Tests: Make as many tests as possible async.

In general, tests that match these criteria can be made async:

- Doesn't use real Cachex.
- Doesn't write to the Config / Application Environment.
- Uses Mock. Using Mox is fine.
- Uses the streamer.

test/pleroma/html_test.exs

@@ -6,7 +6,7 @@ defmodule Pleroma.HTMLTest do
   alias Pleroma.HTML
   alias Pleroma.Object
   alias Pleroma.Web.CommonAPI
-  use Pleroma.DataCase
+  use Pleroma.DataCase, async: true
 
   import Pleroma.Factory