REGTEST: add a test for log-backend used as a log target

REGTEST: add a test for log-backend used as a log target

This regtest declares and uses 3 log backends, one of which has TCP syslog
servers declared in it and other ones UDP syslog servers.

Some tests aims at testing log distribution reliability by leveraging the
log-balance hash algorithm with a key extracted from the request URL, and
the dummy vtest syslog servers ensure that messages are sent to the
correct endpoint. Overall this regtest covers essential parts of the log
message distribution and log-balancing logic involved with log backends.

It also leverages the log-forward section to perform the TCP->UDP
translation required to test UDP endpoints since vtest syslog servers
work in UDP mode.

Finally, we have some tests to ensure that the server queuing/dequeuing
and failover (backup) logics work properly.