If we make a package uninstallable, do not queue tests for it. This prevents early triggering of reverse-(test-)deps for transitions. If we have not queued the test because of this, block the migration if the test could itself block due to having passed before.
Add two tests, one to verify the new behaviour, and another to verify that when the rdep does become installable (e.g. when it is rebuilt), we then do trigger the test.