Support excluding sources from coverage

Add support for specifying a list of regular expressions matching source files the tests in the database are not supposed to cover. Put it into the optional not_sources property in the top index.yaml.

Remove the matching source files from the ones retrieved from patches to be tested, before matching tests to them.

Add support for specifying --eligible option to kpet patch list-files, which omits the files matching not_sources from the output.

Concerns https://gitlab.com/cki-project/kernel-tests/-/issues/820