pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/de3de0afd7da7b432e219aa38bde248fc5c5206a

Modified Files
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src/test/perl/PostgresNode.pm | 29 ++++++++++++++++++-----------
src/test/recovery/t/007_sync_rep.pl | 21 ++-------------------
2 files changed, 20 insertions(+), 30 deletions(-)