Update unit tests for stream_output to match the behaviour of a docker-py response.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

tests/integration/legacy_test.py

@@ -1,8 +1,8 @@
 import unittest
 
 from docker.errors import APIError
-from mock import Mock
 
+from .. import mock
 from .testcases import DockerClientTestCase
 from compose import legacy
 from compose.project import Project
@@ -66,7 +66,7 @@ class UtilitiesTestCase(unittest.TestCase):
         )
 
     def test_get_legacy_containers(self):
-        client = Mock()
+        client = mock.Mock()
         client.containers.return_value = [
             {
                 "Id": "abc123",