Replace the custom DNS server used in the "upforwd" system test with new
code based on the isctest.asyncserver module. The ans4 server currently
used in that test is a copy of bin/tests/system/ans.pl modified to
receive queries over UDP and TCP without ever responding to any of them.
Closes #5012
Merge branch '5012-upforwd-asyncserver' into 'main'
See merge request isc-projects/bind9!10283