Second try at stabilizing query plans in rowsecurity regression test.

Second try at stabilizing query plans in rowsecurity regression test.

This reverts commit 5cdf25e16843dff33dbc2ddc02941458032e3ad4,
which was almost immediately proven insufficient by the buildfarm.

On second thought, the tables involved are not large enough that
autovacuum or autoanalyze would notice them; what seems far more
likely to be the culprit is the database-wide "vacuum analyze"
in the concurrent gist test.  That thing has given us one headache
too many, so get rid of it in favor of targeted vacuuming of that
test's own tables only.