[gator verify] Add opa test coverage support

[willbeason]

Allow optionally returning coverage of opa code that "opa test" uses for the "--coverage" option

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[ritazh]

As discussed on 8/3/3022 community call, opa test has a --coverage flag to report coverage done by opa test.

[sozercan]

this should be done in a way that enables more extensibility. it will be a new features, so removing it from beta milestone