Add tests for DNSSEC sign statistics

Add tests for DNSSEC sign statistics

This adds tests to the statschannel system test for testing if
the dnskey sign operation counters are incremented correctly.

It tests three cases:

1. A zone maintenance event where all the signatures that are about
   to expire are resigned.
2. A dynamic update event where the new RR and other relevant records
   (SOA, NSEC) are resigned.
3. Adding a standby key, that means the DNSKEY and SOA RRset are
   resigned.

(cherry picked from commit a8750a8805c2f3055059e647ad24ee4e289b26b6)