pgsql: Make PostgresNode.pm's poll_query_until() more chatty aboutfail

Make PostgresNode.pm's poll_query_until() more chatty about failures.

Reporting only the stderr is unhelpful when the problem is that the
server output we're getting doesn't match what was expected.  So we
should report the query output too; and just for good measure, let's
print the query we used and the output we expected.

Back-patch to 9.5 where poll_query_until was introduced.

Discussion: https://postgr.es/m/17913.1539634756@sss.pgh.pa.us

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5f920def84a0df35e3a455068ba0ee0df0f45493

Modified Files
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src/test/perl/PostgresNode.pm | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)