el6_tests: fix toolTests to use VdsmTestCase so that assertIn is available

ovirt-3.5 backport note: at 3.5 VdsmTestCase is imported from
testrunner and not testlib.

Change-Id: Ibb564320db9db1b3349c7fbb87bc63a8a044c6b2
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31968
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32103
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Tested-by: mooli tayer <mtayer@redhat.com>