[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/sources_test.exs

@@ -1,6 +1,6 @@
 defmodule Pinchflat.SourcesTest do
   use Pinchflat.DataCase
-  import Mox
+
   import Pinchflat.TasksFixtures
   import Pinchflat.MediaFixtures
   import Pinchflat.ProfilesFixtures
@@ -18,8 +18,6 @@ defmodule Pinchflat.SourcesTest do
 
   @invalid_source_attrs %{name: nil, collection_id: nil}
 
-  setup :verify_on_exit!
-
   describe "schema" do
     test "source_metadata is deleted when the source is deleted" do
       source =