net tests: more combinations mtu combinations

The tests were splitted from the former functional fix in order to
expedite the merge of the latter one.

Change-Id: Id7a1f5b628261ae4ebf400494bcdb2939a9cd535
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19230