Enable running rspec in parallel based on a given list of test files

We are almost blocked in test execution because rspec fail-fast is not parallel. If we were to move towards dynamic mapping, the number of tests that need to run would increase.

To unblock, we'd need to be able to parallelise this job.

Challenges:

• rspec fail-fast calls rspec with list of spec files. This may not be supported by knapsack, as knapsack calls rspec with an internally generated list of test files https://github.com/ArturT/knapsack/blob/387181ed7a459557770514c676771eff27dc3e1f/lib/knapsack/runners/rspec_runner.rb#L15. There is KNAPSACK_PRO_TEST_FILE_LIST but it is in KnapsackPro https://github.com/KnapsackPro/knapsack_pro-ruby#how-to-run-a-specific-list-of-test-files-or-only-some-tests-from-test-file