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
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0a576cd2a9ab8267720709481cdbbd5e06988821

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