Fix warnings in tests, treat warnings as errors in CI.

The warnings revealed two bad tests, the code still worked but the test
didn't actually test for it. Activating this for CI to prevent issues
like these in the future.

test/pleroma/web/activity_pub/transmogrifier/emoji_react_handling_test.exs

@@ -65,7 +65,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier.EmojiReactHandlingTest do
     object = Object.get_by_ap_id(data["object"])
 
     assert object.data["reaction_count"] == 1
-    assert match?([[emoji, _]], object.data["reactions"])
+    assert match?([[^emoji, _]], object.data["reactions"])
   end
 
   test "it reject invalid emoji reactions" do

test/pleroma/web/mastodon_api/update_credentials_test.exs

@@ -375,7 +375,9 @@ defmodule Pleroma.Web.MastodonAPI.UpdateCredentialsTest do
           "pleroma_background_image" => new_background_oversized
         })
 
-      assert user_response = json_response_and_validate_schema(res, 413)
+      assert %{"error" => "File is too large"} == json_response_and_validate_schema(res, 413)
+
+      user = Repo.get(User, user.id)
       assert user.background == %{}
 
       clear_config([:instance, :upload_limit], upload_limit)