increased flakiness in `predicate-rum` test after 1.3.15 release

[thesuhas]

Hi,

After the latest 1.3.15 release, we're seeing increased flakiness in the predicate-rum test. This is not limited to PG18, as we are also seeing failures in PG16 and PG17.

--- /expected/predicate-rum.out	2025-10-23 11:56:59.000000000 +0000
+++ /results/predicate-rum.out	2025-11-07 13:27:13.491494001 +0000
@@ -259,6 +259,7 @@
 step wy2: INSERT INTO rum_tbl(tsv) values('hx');
 step c2: COMMIT;
 step wx1: INSERT INTO rum_tbl(tsv) values('qh');
+ERROR:  could not serialize access due to read/write dependencies among transactions
 step c1: COMMIT;
 
 starting permutation: rxy2 rxy1 wx1 c1 wy2 c2
not ok 1     - predicate-rum                            4384 ms
ok 2         - predicate-rum-2                          5702 ms
1..2
# 1 of 2 tests failed.