tests: Clean up test classes names

Now when the securable tests have their own module, we don't need to use
inner classes. Using top level classes makes the code shorter and easier
to read.

While modifying the names, the "My" prefix was removed, as it adds no
value and annoy the author of this patch.

Finally, a class method in a test class named its first argument "self"
instead of "cls" (detected by pylint).

Change-Id: Ia2156071c5d2842e45e027933e2d51ba98bab1de
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24488
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>