[Housekeeping] Improve test stability (#232)

* Ensured all tests use at least the DataCase module

* addressed one flakey test

* More flakey tests

* Consolidated Mox usage

test/pinchflat/downloading/media_downloader_test.exs

@@ -1,14 +1,12 @@
 defmodule Pinchflat.Downloading.MediaDownloaderTest do
   use Pinchflat.DataCase
-  import Mox
+
   import Pinchflat.MediaFixtures
   import Pinchflat.SourcesFixtures
   import Pinchflat.ProfilesFixtures
 
   alias Pinchflat.Downloading.MediaDownloader
 
-  setup :verify_on_exit!
-
   setup do
     media_item =
       Repo.preload(