Revise test setup

[barryhughes]

• Review and revise our test suite as needed.
• Make it more reliable in CI.
• Make it easier to run locally.

Sometimes, our unit tests fail to run in CI. For whatever reason, this seems to surface in relation to community PRs in particular (example).

Frustratingly, based on my own recent experience, it is also challenging to run locally without jumping through various hoops (refer to the pr-unit-tests.yml workflow to see what I mean: if you are using a modern PHP, extra work is needed) and I would assume that means it is also difficult for community contributors (doubly so since, as already noted, there is a greater chance of CI not working in their case in addition to the challenge of making it run locally).

See also #1213