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/null_cache.ex

@@ -0,0 +1,47 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.NullCache do
+  @moduledoc """
+  A module simulating a permanently empty cache.
+  """
+  @behaviour Pleroma.Caching
+
+  @impl true
+  def get!(_, _), do: nil
+
+  @impl true
+  def put(_, _, _, _ \\ nil), do: {:ok, true}
+
+  @impl true
+  def stream!(_, _), do: []
+
+  @impl true
+  def get(_, _), do: {:ok, nil}
+
+  @impl true
+  def fetch!(_, _, func) do
+    {_, res} = func.()
+    res
+  end
+
+  @impl true
+  def get_and_update(_, _, func) do
+    func.(nil)
+  end
+
+  @impl true
+  def expire_at(_, _, _), do: {:ok, true}
+
+  @impl true
+  def exists?(_, _), do: {:ok, false}
+
+  @impl true
+  def execute!(_, func) do
+    func.(:nothing)
+  end
+
+  @impl true
+  def del(_, _), do: {:ok, true}
+end