tests: Use testrunner.namedTemporaryDir

Some tests were working hard ensuring proper cleanup when a test fails,
instead of using the existing infrastructure.

Change-Id: Idd7a91611a6abd5b88d51b748ce7e0172285875c
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27506
Reviewed-by: Dan Kenigsberg <danken@redhat.com>