tests: use assertIn when applicable

On failure, assertIn(x, y) provides more tangible information than the
plain assertTrue(x in y).

Change-Id: Idf30dae017ba9e028b0adf00e1edab04c3b27c0d
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23038
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>