Update various system tests and add them to default test suite

The stress, dialup and dupsigs tests haven't been executed regularly and became outdated as a result.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 dnssec-dnskey-kskonly.