[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/downloading_helpers_test.exs

@@ -1,7 +1,6 @@
 defmodule Pinchflat.Downloading.DownloadingHelpersTest do
   use Pinchflat.DataCase
 
-  import Mox
   import Pinchflat.MediaFixtures
   import Pinchflat.SourcesFixtures
   import Pinchflat.ProfilesFixtures
@@ -10,8 +9,6 @@ defmodule Pinchflat.Downloading.DownloadingHelpersTest do
   alias Pinchflat.Downloading.DownloadingHelpers
   alias Pinchflat.Downloading.MediaDownloadWorker
 
-  setup :verify_on_exit!
-
   describe "enqueue_pending_download_tasks/1" do
     test "it enqueues a job for each pending media item" do
       source = source_fixture()