Add AXFR regression test for SIG covers preservation

Add AXFR regression test for SIG covers preservation

diff.c rdata_covers() runs on both dns_diff_apply (IXFR, ns/update.c
dynamic updates) and dns_diff_load (AXFR).  After the previous commit
refused SIG and NXT in dynamic updates, the AXFR path remains the
most natural way to drive legacy SIG records into a secondary's zone
DB and regression-gate the rdata_covers() fix.

The test adds ans11 as an AsyncDnsServer primary for a small zone
whose AXFR carries two SIG rdatas at the same owner with different
covered types (A, MX) and different TTLs (600, 1200), and declares
ns6 a secondary of that zone.  With the bug present, dns_diff_load
groups both tuples at typepair (SIG, 0) and the MX-covering record
inherits the first-seen TTL (600); the fix keeps them at (SIG, A)
and (SIG, MX) with their original TTLs.

rndc dumpdb -zones on the secondary is used to inspect stored state
directly, because the wire-level SIG query response merges
same-(owner,type,class) RRs and masks the per-rdataset TTLs.