Attempt to stabilize partition_prune test output.

Disable index-only scan for tests that might report variable results
for "Heap Fetches" statistic due to concurrent transactions affecting
whether all-visible flags can be set.

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f_yjtHDJnDzx1uuR_3D7beDVAkNQfWJhRLA1gvPCzkAhg@mail.gmail.com

src/test/regress/sql/partition_prune.sql

@@ -256,6 +256,11 @@ create table ab_a3_b1 partition of ab_a3 for values in (1);
 create table ab_a3_b2 partition of ab_a3 for values in (2);
 create table ab_a3_b3 partition of ab_a3 for values in (3);
 
+-- Disallow index only scans as concurrent transactions may stop visibility
+-- bits being set causing "Heap Fetches" to be unstable in the EXPLAIN ANALYZE
+-- output.
+set enable_indexonlyscan = off;
+
 prepare ab_q1 (int, int, int) as
 select * from ab where a between $1 and $2 and b <= $3;
 
@@ -581,3 +586,5 @@ explain (analyze, costs off, summary off, timing off)
 select * from boolp where a = (select value from boolvalues where not value);
 
 drop table boolp;
+
+reset enable_indexonlyscan;
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