Do not use Net::DNS::Nameserver in the "serve-stale" system test
Net::DNS versions older than 0.67 respond to queries sent to a Net::DNS::Nameserver even if its ReplyHandler returns undef. This makes the "serve-stale" system test fail as it takes advantage of the newer behavior. Since the latest Net::DNS version available with stock RHEL/CentOS 6 packages is 0.65 and we officially support that operating system, bin/tests/system/serve-stale/ans2/ans.pl should behave consistently for various Net::DNS versions. Ensure that by reworking it so that it does not use Net::DNS::Nameserver.
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