Improve TAP test function PostgresNode::poll_query_until().

Improve TAP test function PostgresNode::poll_query_until().

Add an optional "expected" argument to override the default assumption
that we're waiting for the query to return "t".  This allows replacing
a handwritten polling loop in recovery/t/007_sync_rep.pl with use of
poll_query_until(); AFAICS that's the only remaining ad-hoc polling
loop in our TAP tests.

Change poll_query_until() to probe ten times per second not once per
second.  Like some similar changes I've been making recently, the
one-second interval seems to be rooted in ancient traditions rather
than the actual likely wait duration on modern machines.  I'd consider
reducing it further if there were a convenient way to spawn just one
psql for the whole loop rather than one per probe attempt.

Discussion: https://postgr.es/m/12486.1498938782@sss.pgh.pa.us