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/support/data_case.ex

@@ -65,8 +65,13 @@ defmodule Pleroma.DataCase do
   setup tags do
     :ok = Ecto.Adapters.SQL.Sandbox.checkout(Pleroma.Repo)
 
-    unless tags[:async] do
+    if tags[:async] do
+      Mox.stub_with(Pleroma.CachexMock, Pleroma.NullCache)
+      Mox.set_mox_private()
+    else
       Ecto.Adapters.SQL.Sandbox.mode(Pleroma.Repo, {:shared, self()})
+      Mox.stub_with(Pleroma.CachexMock, Pleroma.CachexProxy)
+      Mox.set_mox_global()
       clear_cachex()
     end