test: switch to native v8 coverage

Switches to using v8's built in coverage support.

Note: these results will get better once https://chromium-review.googlesource.com/c/v8/v8/+/1339119 is landed.

CC: @schuay, @hashseed we're almost over the finish line \o/

TODO:

• update documentation on how to run coverage.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines