Re-organize fixtures and util functions in tests.

Right now everything is shoved in fixtures. Perhaps we could organize them better with `src/test/utils/` and maybe re-organize things to be more consolidated in files. It feels really messy right now on how we organize and store things.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-718)

Start by inventorying the existing fixtures and utility functions under the test setup, then compare them with the proposed src/test/utils/ location. Done means the test helpers are organized into a consolidated, understandable structure without breaking the existing test suite.

Written by the indexing model from the issue text.