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.

config/test.exs

@@ -121,6 +121,8 @@ config :tzdata, :autoupdate, :disabled
 
 config :pleroma, :mrf, policies: []
 
+config :pleroma, :cachex, provider: Pleroma.CachexMock
+
 if File.exists?("./config/test.secret.exs") do
   import_config "test.secret.exs"
 else