FilterTest: tag erratic test

Prevents this failure:

  1) test creating a filter with expires_in (Pleroma.Web.MastodonAPI.FilterControllerTest)
     test/pleroma/web/mastodon_api/controllers/filter_controller_test.exs:52
     Assertion with == failed
     code:  assert response["expires_at"] == expires_at
     left:  "2022-01-22T18:11:12.000Z"
     right: "2022-01-22T18:11:13.000Z"
     stacktrace:
       test/pleroma/web/mastodon_api/controllers/filter_controller_test.exs:72: (test)