Raise AssertionError instead of RuntimeError (#13)

Currently, assertions `assert_has` and `refute_has` raise `RuntimeError`

```
  1) test index shows empty states (Manage.PageLiveTest)
     apps/manage/test/manage/live/page_test.exs:6
     ** (RuntimeError) Could not find any elements with selector "span#bar".

     code: |> assert_has("span#bar", "foo")
     stacktrace:
       (phoenix_test 0.2.1) lib/phoenix_test/assertions.ex:17: PhoenixTest.Assertions.assert_has/3
       test/manage/live/page_test.exs:9: (test)
```

This works, but it's more common for assertion helpers to raise
`ExUnit.AssertionError` instead, as it signals that
there is something wrong with what the test author is asserting vs.
something wrong with the setup as such.

This PR introduces `AssertionError` to the `Assertions` module, and only
there. This is how output of the same test
looks like:

```
  1) test index shows empty states (Manage.PageLiveTest)
     apps/manage/test/manage/live/page_test.exs:6
     Could not find any elements with selector "span#bar".

     code: |> assert_has("span#bar", "foo")
     stacktrace:
       (phoenix_test 0.2.1) lib/phoenix_test/assertions.ex:18: PhoenixTest.Assertions.assert_has/3
       test/manage/live/page_test.exs:9: (test)
```

lib/phoenix_test/assertions.ex

@@ -2,6 +2,7 @@ defmodule PhoenixTest.Assertions do
   @moduledoc false
 
   import ExUnit.Assertions
+  alias ExUnit.AssertionError
 
   alias PhoenixTest.Query
 
@@ -14,18 +15,20 @@ defmodule PhoenixTest.Assertions do
         assert true
 
       {:not_found, []} ->
-        raise """
-        Could not find any elements with selector #{inspect(selector)}.
-        """
+        raise AssertionError,
+          message: """
+          Could not find any elements with selector #{inspect(selector)}.
+          """
 
       {:not_found, elements_matched_selector} ->
-        raise """
-        Could not find element with text #{inspect(text)}.
+        raise AssertionError,
+          message: """
+          Could not find element with text #{inspect(text)}.
 
-        Found other elements matching the selector #{inspect(selector)}:
+          Found other elements matching the selector #{inspect(selector)}:
 
-        #{format_found_elements(elements_matched_selector)}
-        """
+          #{format_found_elements(elements_matched_selector)}
+          """
     end
 
     session
@@ -40,20 +43,22 @@ defmodule PhoenixTest.Assertions do
         refute false
 
       {:found, element} ->
-        raise """
-        Expected not to find an element.
+        raise AssertionError,
+          message: """
+          Expected not to find an element.
 
-        But found an element with selector #{inspect(selector)} and text #{inspect(text)}:
+          But found an element with selector #{inspect(selector)} and text #{inspect(text)}:
 
-        #{format_found_element(element)}
-        """
+          #{format_found_element(element)}
+          """
 
       {:found_many, elements} ->
-        raise """
-        Expected not to find an element.
+        raise AssertionError,
+          message: """
+          Expected not to find an element.
 
-        But found #{Enum.count(elements)} elements with selector #{inspect(selector)} and text #{inspect(text)}:
-        """
+          But found #{Enum.count(elements)} elements with selector #{inspect(selector)} and text #{inspect(text)}:
+          """
     end
 
     session

test/phoenix_test/assertions_test.exs

@@ -2,6 +2,7 @@ defmodule PhoenixTest.AssertionsTest do
   use ExUnit.Case, async: true
 
   import PhoenixTest
+  alias ExUnit.AssertionError
 
   setup do
     %{conn: Phoenix.ConnTest.build_conn()}
@@ -55,7 +56,7 @@ defmodule PhoenixTest.AssertionsTest do
 
       msg = ~r/Could not find any elements with selector "#nonexistent-id"/
 
-      assert_raise RuntimeError, msg, fn ->
+      assert_raise AssertionError, msg, fn ->
         conn |> assert_has("#nonexistent-id", "Main page")
       end
     end
@@ -65,15 +66,17 @@ defmodule PhoenixTest.AssertionsTest do
     } do
       conn = visit(conn, "/page/index")
 
-      msg = """
-      Could not find element with text "Super page".
+      msg =
+        """
+        Could not find element with text "Super page".
 
-      Found other elements matching the selector "h1":
+        Found other elements matching the selector "h1":
 
-      <h1> with content "Main page"
-      """
+        <h1> with content "Main page"
+        """
+        |> ignore_whitespace()
 
-      assert_raise RuntimeError, msg, fn ->
+      assert_raise AssertionError, msg, fn ->
         conn |> assert_has("h1", "Super page")
       end
     end
@@ -83,25 +86,27 @@ defmodule PhoenixTest.AssertionsTest do
     } do
       conn = visit(conn, "/page/index")
 
-      msg = """
-      Could not find element with text "Frodo".
+      msg =
+        """
+        Could not find element with text "Frodo".
 
-      Found other elements matching the selector "#multiple-items":
+        Found other elements matching the selector "#multiple-items":
 
-      <ul> with content:
-      <li>
-        Aragorn
-      </li>
-      <li>
-        Legolas
-      </li>
-      <li>
-        Gimli
-      </li>
+        <ul> with content:
+        <li>
+          Aragorn
+        </li>
+        <li>
+          Legolas
+        </li>
+        <li>
+          Gimli
+        </li>
 
-      """
+        """
+        |> ignore_whitespace()
 
-      assert_raise RuntimeError, msg, fn ->
+      assert_raise AssertionError, msg, fn ->
         conn |> assert_has("#multiple-items", "Frodo")
       end
     end
@@ -139,31 +144,46 @@ defmodule PhoenixTest.AssertionsTest do
     test "raises an error if one element is found", %{conn: conn} do
       conn = visit(conn, "/page/index")
 
-      msg = """
-      Expected not to find an element.
+      msg =
+        """
+        Expected not to find an element.
 
-      But found an element with selector "#title" and text "Main page":
+        But found an element with selector "#title" and text "Main page":
 
-      <h1> with content "Main page"
-      """
+        <h1> with content "Main page"
+        """
+        |> ignore_whitespace()
 
-      assert_raise RuntimeError, msg, fn ->
+      assert_raise AssertionError, msg, fn ->
         conn |> refute_has("#title", "Main page")
       end
     end
 
     test "raises an error if multiple elements are found", %{conn: conn} do
       conn = visit(conn, "/page/index")
 
-      msg = """
-      Expected not to find an element.
+      msg =
+        """
+        Expected not to find an element.
 
-      But found 2 elements with selector ".multiple_links" and text "Multiple links":
-      """
+        But found 2 elements with selector ".multiple_links" and text "Multiple links":
+        """
+        |> ignore_whitespace()
 
-      assert_raise RuntimeError, msg, fn ->
+      assert_raise AssertionError, msg, fn ->
         conn |> refute_has(".multiple_links", "Multiple links")
       end
     end
   end
+
+  # converts a multi-line string into a whitespace-forgiving regex
+  defp ignore_whitespace(string) do
+    string
+    |> String.split("\n")
+    |> Enum.map(&String.trim/1)
+    |> Enum.reject(fn s -> s == "" end)
+    |> Enum.map(fn s -> "\\s*" <> s <> "\\s*" end)
+    |> Enum.join("\n")
+    |> Regex.compile!([:dotall])
+  end
 end