Fix formatting found elements with nested children (#9)

As part of its assertion error messages, PhoenixTest outputs HTML
elements that match the query selector but do not
contain the requested text. Currently it only works for elements that
only contain one child, and breaks otherwise. It
is sometimes useful to assert that a word will be present in a complex
structure (maybe generated, like the modal popup
from `PetalComponents`) without coupling the code too much to the
internal DOM of the structure.

Proposal: allow complex structures but not assuming only one child by
default, and use `Floki.raw_html(content, pretty: true)` to render the
error message.

lib/phoenix_test/assertions.ex

@@ -64,6 +64,10 @@ defmodule PhoenixTest.Assertions do
   end
 
   defp format_found_element({tag, _attrs, [content]}) do
-    "<#{tag}> with content #{inspect(content)}"
+    "<#{tag}> with content \"#{Floki.raw_html(content)}\""
+  end
+
+  defp format_found_element({tag, _attrs, content}) when is_list(content) do
+    "<#{tag}> with content:\n#{Floki.raw_html(content, pretty: true)}"
   end
 end

test/phoenix_test/assertions_test.exs

@@ -77,6 +77,34 @@ defmodule PhoenixTest.AssertionsTest do
         conn |> assert_has("h1", "Super page")
       end
     end
+
+    test "raises error if element cannot be found and selector matches a nested structure", %{
+      conn: conn
+    } do
+      conn = visit(conn, "/page/index")
+
+      msg = """
+      Could not find element with text "Frodo".
+
+      Found other elements matching the selector "#multiple-items":
+
+      <ul> with content:
+      <li>
+        Aragorn
+      </li>
+      <li>
+        Legolas
+      </li>
+      <li>
+        Gimli
+      </li>
+
+      """
+
+      assert_raise RuntimeError, msg, fn ->
+        conn |> assert_has("#multiple-items", "Frodo")
+      end
+    end
   end
 
   describe "refute_has/3" do