image-download: tests to catch stray output

Add unit tests to ensure that any stray output (eg print
statements) during image-download cause a test failure.

Regression test for bug 1488914.

Change-Id: Ic19ba5693d059bf7c283702e7c333672a878a1a1
Partial-bug: 1488914

glanceclient/common/utils.py

@@ -283,10 +283,8 @@ def save_image(data, path):
     :param path: path to save the image to
     """
     if path is None:
-        if six.PY3:
-            image = sys.stdout.buffer
-        else:
-            image = sys.stdout
+        image = getattr(sys.stdout, 'buffer',
+                        sys.stdout)
     else:
         image = open(path, 'wb')
     try: