REGTESTS: http-messaging: give each frontend its own syslog server

REGTESTS: http-messaging: give each frontend its own syslog server

The two frontends of this test were logging to the same syslog server
and their two messages were expected in order. A stream is logged when
it ends, which may be after the next client has been served by the
other frontend, so that order is not guaranteed. This is the same race
which was caught in the raw hello test when the suite runs in parallel,
found here by inspection rather than by a failure.