Avoid rare race condition in privileges.sql regression test.

We created a temp table, then switched to a new session, leaving
the old session to clean up its temp objects in background.  If that
took long enough, the eventual attempt to drop the user that owns
the temp table could fail, as exhibited today by sidewinder.
Fix by dropping the temp table explicitly when we're done with it.

It's been like this for quite some time, so back-patch to all
supported branches.

Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=sidewinder&dt=2018-10-16%2014%3A45%3A00