Update kasp test with CDNSKEY checks

Update kasp test with CDNSKEY checks

Add checks to the kasp system test to verify CDNSKEY publication.
This test is not entirely complete, because when there is a CDNSKEY
available but there should not be one for KEY N, it is hard to tell
whether the existing CDNSKEY actually belongs to KEY N or another
key.

The check works if we expect a CDNSKEY although we cannot guarantee
that the CDNSKEY is correct: The test verifies existence, not
correctness of the record.