create_index test fails when synchronous_commit = off @ master

Hi hackers,

I noticed that create_index test (make installcheck) fails on my laptop
because different query plans are chosen:

- QUERY PLAN
--------------------------------------------------------
- Index Only Scan using tenk1_unique1 on tenk1
- Index Cond: (unique1 = ANY ('{1,42,7}'::integer[]))
-(2 rows)
+ QUERY PLAN
+-------------------------------------------------------------------
+ Sort
+ Sort Key: unique1
+ -> Bitmap Heap Scan on tenk1
+ Recheck Cond: (unique1 = ANY ('{1,42,7}'::integer[]))
+ -> Bitmap Index Scan on tenk1_unique1
+ Index Cond: (unique1 = ANY ('{1,42,7}'::integer[]))
+(6 rows)

...

- QUERY PLAN
--------------------------------------------------------
- Index Only Scan using tenk1_thous_tenthous on tenk1
- Index Cond: (thousand < 2)
- Filter: (tenthous = ANY ('{1001,3000}'::integer[]))
-(3 rows)
+ QUERY PLAN
+--------------------------------------------------------------------------------------
+ Sort
+ Sort Key: thousand
+ -> Index Only Scan using tenk1_thous_tenthous on tenk1
+ Index Cond: ((thousand < 2) AND (tenthous = ANY
('{1001,3000}'::integer[])))
+(4 rows)

Investigation showed that it happens only with `synchronous_commit = off`.
Only the master branch is affected, starting from cc50080a82. This is a
debug build. I'm running MacOS Monterey 12.2.1 @ x64.

I didn't investigate further. Do we assume that `make installcheck` suppose
to pass with a different postgresql.conf options?

--
Best regards,
Aleksander Alekseev