Fix a test case that assumes busybox image id.

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

tests/integration/testcases.py

@@ -1,6 +1,8 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
+from docker import errors
+
 from .. import unittest
 from compose.cli.docker_client import docker_client
 from compose.config.config import ServiceLoader
@@ -9,6 +11,13 @@ from compose.progress_stream import stream_output
 from compose.service import Service
 
 
+def pull_busybox(client):
+    try:
+        client.inspect_image('busybox:latest')
+    except errors.APIError:
+        client.pull('busybox:latest', stream=False)
+
+
 class DockerClientTestCase(unittest.TestCase):
     @classmethod
     def setUpClass(cls):