PRX-55: Run tests in parallel
There are two kinds of test fixtures (classes)
- fully parallel (no setup/teardown)
- isolated from other tests (tests within the fixture cannot be parallelized but the fixture can run in parallel with others.)
The former gets the `[Parallelizable(ParallelScope.Fixtures | ParallelScope.Self)]` annotation; the latter only `[Parallelizable(ParallelScope.Fixtures)]`