Suppress occasional failures in brin regression test.

brin.sql included a call of brin_summarize_new_values(), and expected
it to always report exactly 5 summarization events.  This failed sometimes
during parallel regression tests, as a consequence of the database-wide
VACUUM in gist.sql getting there first.  The most future-proof way
to avoid variation in the test results is to forget about using
brin_summarize_new_values() and just do a plain "VACUUM brintest",
which will exercise the same code anyway.

Having done that, there's no need for preventing autovacuum on brintest;
doing so just reduces the scope of test coverage, so let's not.