Commit 6271fceb changed PostgresNode.pm to force log_min_messages = debug1
in all TAP tests, without any discussion and without a concrete need for
it.  This makes some of the TAP tests noticeably slower (although much of
that may be due to poorly-written regexes), and for certain it's bloating
the buildfarm logs.  Revert the change.

Discussion: https://postgr.es/m/32459.1525657786@sss.pgh.pa.us