contrib/pg_visibility fails regression under CLOBBER_CACHE_ALWAYS

husky just reported $SUBJECT:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=husky&dt=2021-06-05%2013%3A42%3A17

and I find I can reproduce that locally:

diff -U3 /home/postgres/pgsql/contrib/pg_visibility/expected/pg_visibility.out /home/postgres/pgsql/contrib/pg_visibility/results/pg_visibility.out
--- /home/postgres/pgsql/contrib/pg_visibility/expected/pg_visibility.out 2021-01-20 11:12:24.854346717 -0500
+++ /home/postgres/pgsql/contrib/pg_visibility/results/pg_visibility.out 2021-06-06 22:12:07.948890104 -0400
@@ -215,7 +215,8 @@
0 | f | f
1 | f | f
2 | t | t
-(3 rows)
+ 3 | t | t
+(4 rows)

select * from pg_check_frozen('copyfreeze');
t_ctid
@@ -235,7 +236,8 @@
0 | t | t
1 | f | f
2 | t | t
-(3 rows)
+ 3 | t | t
+(4 rows)

select * from pg_check_frozen('copyfreeze');
t_ctid

The test cases that are failing date back to January (7db0cd2145f),
so I think this is some side-effect of a recent commit, but I have
no idea which one.

regards, tom lane