Minor cleanups in test_volume

Fix incorrect parameter order for mock decorators.  This
just happened to work because they were used in the same way
in test_reserve_volume_success().

Remove incorrect comments.  These seem to mix up 'error' versus
'error_deleting' for expected states -- but the assertEqual args
are sufficiently explanatory by themselves.

Change-Id: I763265b8365d3b2fcece94b14639a1bed7b6a848
Merge "Minor cleanups in test_volume"