chg: test: Use isctest.asyncserver in the "zero" test

The original `ans.pl` server was a copy of the one in `fetchlimit`, so
there are some changes:

- The server now only responds with A replies (which is the only thing
  needed).
- The incrementing of the IP address goes beyond the least significant
  octet (so, after 192.0.2.255 it will yield 192.0.3.0).

Merge branch 'stepan/zero-asyncserver' into 'main'

See merge request isc-projects/bind9!10597