test: better way to run test/test_all async

Author: rNoz

test/support/image_test_case.ex

@@ -1,6 +1,6 @@
 defmodule ImageTestCase do
   @moduledoc false
-  use ExUnit.CaseTemplate
+  use ExUnit.CaseTemplate, async: true
 
   @doc "Reads an image giving a relative path to test/fixtures/images"
   def read_image(rel_path) do
@@ -10,8 +10,6 @@ defmodule ImageTestCase do
 
   using do
     quote do
-      use ExUnit.Case, async: true
-
       def read_image(rel_path), do: ImageTestCase.read_image(rel_path)
     end
   end