pgsql: Stabilize recovery conflict count checks in 031_recovery_conflic

Stabilize recovery conflict count checks in 031_recovery_conflict.pl

Buildfarm members olingo and adder reported failures in
031_recovery_conflict.pl where a recovery conflict was counted twice.
olingo saw the snapshot conflict count reach 2 instead of 1, while adder
saw the same for the buffer pin conflict count.

The startup process can signal a standby backend again while it is
reporting a recovery-conflict FATAL error. Each processed conflict can
increment the corresponding statistic. Thus, even when the expected
conflict occurs, its counter can exceed 1. The existing poll for exactly 1
can then time out, and the exact check of the aggregate count can fail as
well.

Fix this by polling for a positive count for each conflict type and
accepting an aggregate count of at least the expected number.

Backpatch to v17, where this test was enabled.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/421c0aee-84c8-4c07-b4b9-263095479755%40gmail.com
Backpatch-through: 17

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/de34b14fcb0168395ca7f1990038f1db06fe1697

Modified Files
--------------
src/test/recovery/t/031_recovery_conflict.pl | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)