teach rebaseline_server to generate diffs of rendered SKPs

gm/rebaseline_server/base_unittest.py

 #!/usr/bin/python
 
 """
 Copyright 2014 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 A wrapper around the standard Python unittest library, adding features we need
 for various unittests within this directory.
 """
 
 # System-level imports.
 import os
 import sys
 
 PARENT_DIR = os.path.abspath(os.path.dirname(__file__))
 TRUNK_DIR = os.path.abspath(os.path.join(PARENT_DIR, os.pardir, os.pardir))
 
 # Import the superclass base_unittest module from the tools dir.
+#
+# TODO(epoger): If I don't put this at the beginning of sys.path, the import of
+# tests.base_unittest fails.  That's bad.  I need to come up with a cleaner way
+# of doing this... I think this will involve changing how we import the "boto"
+# library in gs_utils.py, within the common repo.
 TOOLS_DIR = os.path.join(TRUNK_DIR, 'tools')
-if TOOLS_DIR not in sys.path:
-  sys.path.append(TOOLS_DIR)
+if TOOLS_DIR != sys.path[0]:
+  sys.path.insert(0, TOOLS_DIR)
 import tests.base_unittest as superclass_module
 
 
 class TestCase(superclass_module.TestCase):
 
   def __init__(self, *args, **kwargs):
     super(TestCase, self).__init__(*args, **kwargs)
     # Some of the tests within this package want their output validated,
     # so we declare where the expected and actual output will be.
     self._testdata_dir = os.path.join(PARENT_DIR, 'testdata')
 
 def main(*args, **kwargs):
   superclass_module.main(*args, **kwargs)