tests: assertIsNone is not available on Python 2.6

This patch fixes the build on jenkins EL6 slaves.

Change-Id: I4f68576ebb295af2dbd567da785d97eb06fd8a55
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33911
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>