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).
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