dupsigs tests haven't been executed regularly and became outdated as a result. They have been revived and updated and can now be made part of the default test suite.
With these changes, all system tests are executed in CI -- let's not add any more tests which aren't executed by default in the future.
Note for 9.16 backport: selected parts of the
dupsigs commit shouldn't be backported, since 9.16 still has the old default for