feat: add very_good test report_on

[mtwichel]

As a developer, I would like to use the very_good_cli to test my dart_frog applications. However, since a bulk of the code in dart_frog lives outside of the lib folder, it will not be caught by codecov and thus isn't a workable solution.

I believe this could be solved if very_good test had an optional --report-on parameter that worked exactly like dart pub global run coverage:format_coverage --report-on works, where you can specify which directories you would like in the coverage report.

Thank you for your consideration! 💙

[alestiago]

May be related to https://github.com/VeryGoodOpenSource/very_good_coverage/issues/34