REGTEST: resolvers: cover UDP to TCP fallback

REGTEST: resolvers: cover UDP to TCP fallback

Add a Python DNS fixture listening on UDP and TCP on the same port.
It returns an empty truncated response over UDP and a complete response
larger than 1232 bytes over TCP.

The fixture only uses the Python standard library. Tag the test as
devel because it depends on an external interpreter. The feature check
skips it when python3 is unavailable.

Cover default and family-prefixed fallback, late A responses after an
A/AAAA switch, complete UDP replies, invalid questions, explicit
UDP-only and TCP-only nameservers, TCP connection failures, and SRV
updates. A TCP-only truncated SRV response contains usable records and
exercises partial SRV processing without another fallback.

Check the resolver CLI and statistics counters as well as the
Prometheus metric.