Improve assert_reply error messages (#19)

Currently the error assertions happen directly on _all_ the messages
that the channel has received, when we only very specifically care about
the specific reply to our message.

This PR splits that assertion into two steps, the first step gets the
reply, and the second step asserts on the expected status and reply.
They are asserted together so that when there's an error you get
detailed information about what the error payload is, otherwise your
initial test failure may simply tell you something like ":ok does not
equal :error"

lib/channel_spec/testing.ex

@@ -113,7 +113,8 @@ defmodule ChannelSpec.Testing do
            ) do
     quote location: :keep do
       socket = Process.get(unquote(ref))
-      assert_reply(unquote(ref), unquote(status), reply = unquote(reply))
+      assert_reply(unquote(ref), status, reply)
+      assert {unquote(status), unquote(reply)} = {status, reply}
 
       normalized_reply = reply |> Jason.encode!() |> Jason.decode!()