pg_logicalinspect: Stabilize isolation tests.

pg_logicalinspect: Stabilize isolation tests.

The previous isolation tests did not account for the possibility that
the background writer or the checkpointer could write a RUNNING_XACTS
record, which could cause logical decoding to produce more logical
snapshots than expected.

This commit modifies the isolation tests to verify that at least one
logical snapshot contains the expected number of committed or ongoing
catalog-change transactions.

Per buildfarm member skink.

Reported-by: Andres Freund <andres@anarazel.de>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/5qbxud4pvnvmtuoi7weiizm5hmumxaeohx4vztfhrwlfhyz6rj@buh4435mllwo