tests: Ensure cleanup when tests fail

Some tests were using an optimstic approach:

    create_temporary_stuff()
    self.assertTrue(maybe_false)
    cleanup_temporary_stuff()

Other tests, testing directory cleanup code, were not trying to clean up
at all, assuming that the code works.

This patch ensure proper cleanup of temporary files when test fails.

Change-Id: If81cd46b37dd135d8584fde996d0d1d3f8f550a6
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27505
Reviewed-by: Dan Kenigsberg <danken@redhat.com>