pgsql: Close yet another race condition in replication slot test code

Close yet another race condition in replication slot test code

Buildfarm shows that this test has a further failure mode when a
checkpoint starts earlier than expected, so we detect a "checkpoint
completed" line that's not the one we want.  Change the config to try
and prevent this.

Per buildfarm

While at it, update one comment that was forgotten in commit
d18e75664a2f.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20210729.162038.534808353849568395.horikyota.ntt@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ce197e91d02ca38ac1b956a3e0c11596099e7893

Modified Files
--------------
src/backend/access/transam/xlog.c         |  2 +-
src/test/recovery/t/019_replslot_limit.pl | 14 ++++++++++++--
2 files changed, 13 insertions(+), 3 deletions(-)