volume: Re-add accidentally deleted test

This is essentially a partial revert of change
I94aa7a9824e44f9585ffb45e5e7637b9588539b4, which removed some deprecated
commands like 'openstack snapshot *' in favour of 'openstack volume
snapshot *'. Unfortunately the latter appeared to have no test coverage
and were relying on tests for the former to validate behavior. Re-add
the tests removed back then.

Change-Id: Ib2cd975221034c8997d272d43cfb18acefc319fe
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

Author: stephenfin