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.

lib/pleroma/emoji/pack.ex

@@ -417,7 +417,7 @@ defmodule Pleroma.Emoji.Pack do
     ttl_per_file = Pleroma.Config.get!([:emoji, :shared_pack_cache_seconds_per_file])
     overall_ttl = :timer.seconds(ttl_per_file * Enum.count(files))
 
-    @cachex.put!(
+    @cachex.put(
       :emoji_packs_cache,
       pack.name,
       # if pack.json MD5 changes, the cache is not valid anymore

lib/pleroma/web/admin_api/controllers/media_proxy_cache_controller.ex

@@ -40,7 +40,7 @@ defmodule Pleroma.Web.AdminAPI.MediaProxyCacheController do
 
   defp fetch_entries(params) do
     MediaProxy.cache_table()
-    |> @cachex.stream!(@cachex.Query.create(true, :key))
+    |> @cachex.stream!(Cachex.Query.create(true, :key))
     |> filter_entries(params[:query])
   end