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/cachex_proxy.ex

@@ -0,0 +1,40 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.CachexProxy do
+  @behaviour Pleroma.Caching
+
+  @impl true
+  defdelegate get!(cache, key), to: Cachex
+
+  @impl true
+  defdelegate stream!(cache, key), to: Cachex
+
+  @impl true
+  defdelegate put(cache, key, value, options), to: Cachex
+
+  @impl true
+  defdelegate put(cache, key, value), to: Cachex
+
+  @impl true
+  defdelegate get_and_update(cache, key, func), to: Cachex
+
+  @impl true
+  defdelegate get(cache, key), to: Cachex
+
+  @impl true
+  defdelegate fetch!(cache, key, func), to: Cachex
+
+  @impl true
+  defdelegate expire_at(cache, str, num), to: Cachex
+
+  @impl true
+  defdelegate exists?(cache, key), to: Cachex
+
+  @impl true
+  defdelegate del(cache, key), to: Cachex
+
+  @impl true
+  defdelegate execute!(cache, func), to: Cachex
+end