Due to the way the "mirror" system test is set up, it is impossible for
the "verify-unsigned" and "verify-untrusted" zones to contain any serial
number other than the original one present in ns2/verify.db.in.  Thus,
using presence of a different serial number in the SOA records of these
zones as an indicator of problems with mirror zone verification is
wrong.  Look for the original zone serial number instead as that is the
one that will be returned by ns3 if one of the aforementioned zones is
successfully verified.