test_pg_dump missing cleanup actions

Hi Stephen,

While hacking another patch, I have noticed that triggerring multiple
times in a row installcheck on test_pg_dump results in a failure because
it is missing clean up actions on the role regress_dump_test_role.
Roles are shared objects, so I think that we ought to not let traces of
it when doing any regression tests on a running instance.

Attached is a patch to clean up things.

Thanks,
--
Michael