Tests: use Mv.Fixtures, fix warnings, Credo TODO disable

- Policy tests: use Fixtures where applicable; create_custom_field() fix in custom_field_value.
- Replace unused actor with _actor, remove unused alias Accounts in policy tests.
- profile_navigation_test: disable Credo for intentional TODO comment.

test/mv_web/live/profile_navigation_test.exs

@@ -61,6 +61,7 @@ defmodule MvWeb.ProfileNavigationTest do
     end
 
     @tag :skip
+    # credo:disable-for-next-line Credo.Check.Design.TagTODO
     # TODO: Implement user initials in navbar avatar - see issue #170
     test "shows user initials in avatar", %{conn: conn} do
       # Setup: Create and login a user