Support --only-group command-line option

[ilyazub]

parallel_tests has an option to distribute tests across multiple CI workers (eg., strategy.matrix for GitHub Actions.)

For instance, bundle exec parallel_test test -n 6 --only-group 5,6.

I want to be able to do the same with turbo_tests: bundle exec turbo_tests -n6 --only-group 6.

[jgrau]

I know this is old but I just want to mention that I would really love this too. If there is any sort of workaround or alternative solution I'd love to hear about it :)