* One could also block commits to that module on the basis that it's
impossible to know if they are good since they don't go through that
module's tests. But that would probably impact development too much.
This might be reasonable for modules with active maintainers only - if a maintainer is reviewing and approving MRs for the component they should be willing to fix the CI.
* Systematically CC a module's maintainer on bugs about test failures.
* Also CC the "Other knowledgeable persons" if any?
I see no reason not to do that. Again, maintainers should be willing to fix the CI. If it's really unfixable, they can make a case for excluding the test unit. "Other knowledgeable persons" may not have an expectation to fix it but will likely want to know anyway.