fix: test: Fix the nslookup system test

The nslookup system test checks the count of resolved addresses in
the CNAME tests using a 'grep' match on the hostname, and ignoring
lines containing the 'canonical name' string. In order to protect
the check from intermittent failures like the 'address in use' warning
message, which then automatically resolves after a retry, edit the
'grep' matching string to also ignore the comments (as the mentioned
warning message is a comment which contains the hostname).

Closes #4948

Merge branch '4948-nslookup-test-fix' into 'main'

See merge request isc-projects/bind9!9523