tests: split SampleWindow tests

Reorganize and rationalize the unit tests after
the introduction of SampleWindow class.

Change-Id: I74f0005eae5466e083002f0d871bcf1fbec54fef
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34512
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>