The test disconnects, and immediately reconnects and checks that temp
table from the previous session has been removed. However, the old backend
removes the tables only after the disconnection, so there's a race
condition here: the new session might query pg_tables and still see the
table, if the old session was not fast enough to remove it first.

Add a two-second sleep, to make the race condition less likely to happen.
This is not guaranteed to avoid the race altogether, but I don't see an
easy way to wait for the specific event that the old backend has exited.

A sleep in a test that doesn't run in parallel with other tests is a bit
annoying, as it directly makes the whole regression suite slower. To
compensate for that, move the test to run in parallel with the other
nearby bfv_* tests. Those other tests don't use temp tables, so it still
works.

Also, modify the validation query in the test that checks that there are
no temp tables, to return whole rows, not just COUNT(*). That'll make
analyzing easier, if this test fails.