Fix errors in volume set/unset image properties unit tests

These errors may caused by copy-paste, we should specify
a fake return value for get() method but not create() method.
Create() mothod will not called in set/unset command. After
specifing the return value of get(), we can use the fake volume
in the assert.

Change-Id: I1e0df4c28ece373168788c396e9082d565e36cc7

Author: Huanxuan Ao

openstackclient/tests/volume/v2/test_volume.py

@@ -841,7 +841,7 @@ def test_volume_set_image_property(self):
         # returns nothing
         self.cmd.take_action(parsed_args)
         self.volumes_mock.set_image_metadata.assert_called_with(
-            self.volumes_mock.get().id, parsed_args.image_property)
+            self.new_volume.id, parsed_args.image_property)
 
     def test_volume_set_state(self):
         arglist = [
@@ -919,7 +919,7 @@ def setUp(self):
         super(TestVolumeUnset, self).setUp()
 
         self.new_volume = volume_fakes.FakeVolume.create_one_volume()
-        self.volumes_mock.create.return_value = self.new_volume
+        self.volumes_mock.get.return_value = self.new_volume
 
         # Get the command object to set property
         self.cmd_set = volume.SetVolume(self.app, None)
@@ -963,4 +963,4 @@ def test_volume_unset_image_property(self):
         self.cmd_unset.take_action(parsed_args_unset)
 
         self.volumes_mock.delete_image_metadata.assert_called_with(
-            self.volumes_mock.get().id, parsed_args_unset.image_property)
+            self.new_volume.id, parsed_args_unset.image_property)