Stabilize "delzsk.example" zone checks

When a zone is converted from NSEC to NSEC3, the private record at zone apex indicating that NSEC3 chain creation is in progress may be removed during a different (later) zone_nsec3chain() call than the one which adds the NSEC3PARAM record. The "delzsk.example" zone check only waits for the NSEC3PARAM record to start appearing in dig output while private records at zone apex directly affect "rndc signing -list" output. This may trigger false positives for the "autosign" system test as the output of the "rndc signing -list" command used for checking ZSK deletion progress may contain extra lines which are not accounted for. Ensure the private record is removed from zone apex before triggering ZSK deletion in the aforementioned check.

Also future-proof the ZSK deletion progress check by making it only look at lines it should care about.

See #928