Other suggestions I got:
* When a test fails systematically in the GitLab CI, stop running it when testing MRs (i.e. add it to the EXCLUDE_TESTS list). It won't help getting it fixed but at least it will stop it from polluting the CI results.
The downside is we won't notice if the excluded test starts getting even more failures.
* 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.
* Systematically CC a module's maintainer on bugs about test failures.
* Also CC the "Other knowledgeable persons" if any?