Repair unstable regression test.

Repair unstable regression test.

Commit 0c882e52a tried to force table atest12 to have more-accurate-
than-default statistics; but transiently setting default_statistics_target
isn't enough for that, because autovacuum could come along and overwrite
the stats later.  This evidently explains some intermittent buildfarm
failures we've seen since then.  Repair by disabling autovac on this table.

Thanks to David Rowley for correctly diagnosing the cause.

Discussion: https://postgr.es/m/CA+hUKG+OUkQSOUTg=qo=S=fWa_tbm99i7rB7mfbHz1SYm4v-jQ@mail.gmail.com