Tests: Use NullCache for async tests.

Caching can't work in async tests, so for them it is mocked to a
null cache that is always empty. Synchronous tests are stubbed
with the real Cachex, which is emptied after every test.

test/pleroma/reverse_proxy_test.exs

@@ -3,8 +3,7 @@
 # SPDX-License-Identifier: AGPL-3.0-only
 
 defmodule Pleroma.ReverseProxyTest do
-  use Pleroma.Web.ConnCase, async: true
-
+  use Pleroma.Web.ConnCase
   import ExUnit.CaptureLog
   import Mox