Autovacuum-induced regression test instability

In connection with the issue discussed at [1], I tried to run
the core regression tests with extremely aggressive autovacuuming
(I set autovacuum_naptime = 1s, autovacuum_vacuum_threshold = 5,
autovacuum_vacuum_cost_delay = 0).  I found that the timestamp
test tends to fail with diffs caused by unstable row order in
timestamp_tbl.  This is evidently because it does a couple of
DELETEs before inserting the table's final contents; if autovac
comes along at the right time then some of those slots can get
recycled in between insertions.  I'm thinking of committing the
attached patch to prevent this, since in principle such failures
could occur even without hacking the autovac settings.  Thoughts?

			regards, tom lane

[1] https://www.postgresql.org/message-id/15751.1555256860%40sss.pgh.pa.us