[9.20] chg: test: Use isctest.asyncserver in the "cookie" test

Replace the custom DNS servers in the "cookie" system test with new code
based on the isctest.asyncserver module. The convoluted test logic is
split into multiple response handlers which are shared between the two
servers.

Behavior change: returned SOA records have the zone name set properly.

Backport of MR !10691

Merge branch 'backport-stepan/cookie-asyncserver-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11170