pgsql: 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 i

pgsql: 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 i