Tests: Make as many tests as possible async.

In general, tests that match these criteria can be made async:

- Doesn't use real Cachex.
- Doesn't write to the Config / Application Environment.
- Uses Mock. Using Mox is fine.
- Uses the streamer.

test/mix/tasks/pleroma/instance_test.exs

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: AGPL-3.0-only
 
 defmodule Mix.Tasks.Pleroma.InstanceTest do
-  use ExUnit.Case
+  use Pleroma.DataCase, async: true
 
   setup do
     File.mkdir_p!(tmp_path())
@@ -15,15 +15,14 @@ defmodule Mix.Tasks.Pleroma.InstanceTest do
       if File.exists?(static_dir) do
         File.rm_rf(Path.join(static_dir, "robots.txt"))
       end
-
-      Pleroma.Config.put([:instance, :static_dir], static_dir)
     end)
 
     :ok
   end
 
+  @uuid Ecto.UUID.generate()
   defp tmp_path do
-    "/tmp/generated_files/"
+    "/tmp/generated_files/#{@uuid}/"
   end
 
   test "running gen" do