Fix volume revert to snapshot tests

According to the revert to snapshot spec, it should not be
possible to revert a volume to a snapshot after the volume is
extended. This was fixed on [1] but the cinder_tempest_plugin
test still was being considering the old behavior.

[1] https://bugs.launchpad.net/cinder/+bug/1798503

Change-Id: I21d0b2291608922e46278ae5799478a5df0c4904
1 file changed