pgsql: Avoid rare race condition in privileges.sql regression test.

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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/49a1c22889d428558c8e5accb55afa538ef0ca3f

Modified Files
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src/test/regress/expected/privileges.out | 1 +
src/test/regress/sql/privileges.sql | 1 +
2 files changed, 2 insertions(+)