feat(test): add default to --coverage option

[kt3k]

part of #21325

This PR adds default value to --coverage option. If a user only specifies --coverage, it uses coverage directory for coverage data output.

This aligns with the convention of existing JS coverage tooling ecosystem (jest and nyc use the same default directory)

[bartlomieju]

LGTM, while you are looking into these changes, could you verify that internal runtime code coverage is not being written to this directory - I have a feeling we write a lot of unnecessary files for users.

[kt3k]

while you are looking into these changes, could you verify that internal runtime code coverage is not being written to this directory - I have a feeling we write a lot of unnecessary files for users.

Checked with coverage of std/path test case. The urls in json files are all users script's (in this case std source code).

There are multiple json files for a single url, and that causes a large number of json output files, but I think that is the expected behavior.