Use isctest.asyncserver in the "qmin" test

Use isctest.asyncserver in the "qmin" test

Replace custom DNS servers used in the "qmin" system test with new code
based on the isctest.asyncserver module.  The revised code employs zone
files and a limited amount of custom logic, which massively improves
test readability and maintainability, extends logging, and fixes
non-compliant replies sent by some of the custom servers in response to
certain queries (e.g. AA=0 in authoritative empty non-terminal
responses, non-glue address records in ADDITIONAL section).