[9.18] fix: test: Fix 'ans' servers so they respond with consistent answers to NS...

The ANS servers were not to written to handle NS queries at the QNAME, resulting in gratuitous protocol errors that will break tests when NS requests are made for the QNAME: i.e., NXDOMAIN for NS vs data for expected type,  CNAME not being returned for all query types.

Prerequisite for !9155

Closes #5062

Backport of MR !9786

Merge branch 'backport-5062-fix-ans-servers-ns-at-qname-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10065