I have become more or less accidentally the maintainer of a few Drupal modules. Below you'll find resources for them which are not yet available directly from the drupal.org infrastructure.
Resources on contributed modules
To write tests efficiently it's really important to be able to execute them fast and iterate quickly. To do so I'm mostly running PHPUnit locally with xdebug disabled.
To be able to have have up-to-date coverage reports without manual intervention I've used the CI pipeline of a BLT project per module to allow me to work cleanly on contrib changes.
If you do a find/replace on your module path you should be able to import the clover.xml into PHPStorm and get local coverage data.
Caveat: These numbers do not correctly include BrowserTestBase tests, if you have a way of fully tracking that coverage, let me know.