selftest: Add --tmpdir to 'samba-tool gpo create' test

selftest: Add --tmpdir to 'samba-tool gpo create' test

This was the cause of the flakey test, and was only noticed when
multiple different users ran autobuild at the same time on the same
server.

We use shutil.rmtree to wipe the directory before the tests finishes
as required by the TestCaseInTempDir class.

Andrew Bartlett

Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Nov 13 10:50:56 CET 2012 on sn-devel-104