fix(test_server): deprecated ssl_accept

OTP 21 deprecated the `:ssl.ssl_accept/3` function. So, we add a
compile check to determine which function to call. This library
supports Elixir 1.5 and so the OTP support must be present up to OTP 18.

lib/x509/test/server.ex

@@ -4,6 +4,11 @@ defmodule X509.Test.Server do
   """
   use GenServer
 
+  # ssl_accept was deprecated on OTP 21
+  @ssl_function if Code.ensure_loaded?(:ssl) and function_exported?(:ssl, :handshake, 3),
+                  do: :handshake,
+                  else: :ssl_accept
+
   @doc """
   Starts a test server for the given test suite.
 
@@ -78,15 +83,14 @@ defmodule X509.Test.Server do
   end
 
   defp worker(socket, suite, response) do
-    case :ssl.ssl_accept(
-           socket,
-           [
-             active: false,
-             sni_fun: X509.Test.Suite.sni_fun(suite),
-             reuse_sessions: false
-           ] ++ log_opts(),
-           1000
-         ) do
+    opts =
+      [
+        active: false,
+        sni_fun: X509.Test.Suite.sni_fun(suite),
+        reuse_sessions: false
+      ] ++ log_opts()
+
+    case apply(:ssl, @ssl_function, [socket, opts, 1000]) do
       {:ok, ssl_socket} ->
         flush(ssl_socket)
         :ssl.send(ssl_socket, response)