Add --nice flag

[Bo98]

This mimics the same flag available in parallel_tests: https://github.com/grosser/parallel_tests/blob/6e9be6f529b64e5f9c8e4439754bca0b2ef3f2b1/Readme.md?plain=1#L251

[ilyazub]

@Bo98, thanks for all of the improvements! Apologies I didn't review it sooner. I was on vacation and then getting up to speed.

I'll review your PRs, share the feedback, and merge the ones that look good.

[ilyazub]

Thanks for your work @Bo98!

Let's support all of the ParallelTests options instead by calling ParallelTests::CLI.send(:parse_options!, @argv) in TurboTests::CLI#run (ref: #41 (review)).