Proof-of-concept: reduce GitHub Actions unit test durations by removing tox and using test parallelization

[jayaddison]

I removed unittest-parallel from our test suite a couple of years ago in #660, under the justification that it introduced unnecessarily complexity.

However: to support a potential increase in the number of pull requests we run tests for, and based on noticing an increase in the number of continuous integration machine-minutes spent, I'd like to see if we can reduce the duration of our GitHub Actions unit test workflows, and believe that unittest-parallel may be an effective way to do that. So here it is again, although only as a dependency during the relevant workflow.

Depends-upon / includes:

• Tests: refactor: simplify assignment of util test fixtures #1342
• Tests: correctness: ensure that deprecation warnings are always emitted when checking for them #1343
• Tests: prepare test cases at module import-time #1344
• ci: refactor: remove GitHub Actions unit test workflow dependency on 'tox' #1339

[jayaddison]

Note: removal of tox also contributes to the reduction in test runtime here; I introduced it in #650 -- feeling that it provided useful standardisation and isolation of tests.