tests: add an helper schema compliancy check class

This patch extracts the schema compliancy methods
in a self-contained, reusable class.

Change-Id: Iaa07999576c3ace6b5ee7e3e30494bdc9c40d400
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28910
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>