Add test for equality failure

This also revealed that the "test painter" has a missing interface,
yellow/0. Fixed!

[#144]

lib/display/colours.ex

@@ -29,4 +29,5 @@ defmodule Display.Uncoloured do
   def red(str), do: str
   def cyan(str), do: str
   def green(str), do: str
+  def yellow(str), do: str
 end

test/display/failure_test.exs

@@ -14,6 +14,17 @@ defmodule FailureTests do
     assert Failure.format_failure(error) == "Assertion failed in some_file.ex:42\nhola\n"
   end
 
+  test "equality failure" do
+    error = error(%ExUnit.AssertionError{expr: quote(do: :lol == :wat), left: :lol, right: :wat})
+
+    assert Failure.format_failure(error) == """
+                                            Assertion failed in some_file.ex:42
+                                            :lol == :wat
+
+                                            left:  :lol
+                                            right: :wat
+                                            """
+  end
   defp error(error) do
     %{
       error: error,