pgsql: Hopefully make libpq_pipeline's new cancel test more reliable

Hopefully make libpq_pipeline's new cancel test more reliable

The newly introduced cancel test in libpq_pipeline was flaky. It's not
completely clear why, but one option is that the check for "active" was
actually seeing the active state for the previous query. This change
should address any such race condition by first waiting until the
connection is reported as idle.

Author: Jelte Fennema-Nio <me@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQRvmUK5-d68A+cm+fgmfht9Dv2uZ28-qq3QiaF6EAZqPQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1ee910ce437188eab40eddf32dc7d81952e99f82

Modified Files
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src/test/modules/libpq_pipeline/libpq_pipeline.c | 78 +++++++++++++++---------
1 file changed, 49 insertions(+), 29 deletions(-)