Add some basic snapshots listing test

Renames "test_snapshot_create_get_delete" into
"test_snapshot_create_get_list_delete" and adds into it some
get and list assertions. By adding the list test in the existing
create_get_delete sequence we introduce a check without slowing
down the test run as we don't need to recreate the initial
volume/snapshot condition.

Change-Id: I56276619be424c91979bed0133152725ea0993df
Implements: blueprint add-snapshot-tests
Merge "Add some basic snapshots listing test"