rebaseline_server unittests: output tkdiff lines when actuals don't match expectations

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.
@@ skipping 31 matching lines @@
       #    to be validated.
       # 2. So put that validation within tearDown(), which will be called after
       #    every test case!
       #
       # I have confirmed that the test really does fail if this assert is
       # triggered.
       #
       # Ravi notes: if somebody later comes along and adds cleanup code below
       # this assert, then if tests fail, the artifacts will not be cleaned up.
       assert (not different_files), \
-        ('found differing files between actual dir %s and expected dir %s: %s' %
-         (self._output_dir_actual, self._output_dir_expected, different_files))
+        ('found differing files:\n' +
+         '\n'.join(['tkdiff %s %s &' % (
+             os.path.join(self._output_dir_actual, basename),
+             os.path.join(self._output_dir_expected, basename))
+                    for basename in different_files]))
 
   def shortDescription(self):
     """Tell unittest framework to not print docstrings for test cases."""
     return None
 
 
 def create_empty_dir(path):
   """Create an empty directory at the given path."""
   if os.path.isdir(path):
     shutil.rmtree(path)
@@ skipping 27 matching lines @@
   for common_dir in dircmp.common_dirs:
     differing_files.extend(find_different_files(
         os.path.join(dir1, common_dir), os.path.join(dir2, common_dir)))
   return differing_files
 
 
 def main(test_case_class):
   """Run the unit tests within the given class."""
   suite = unittest.TestLoader().loadTestsFromTestCase(test_case_class)
   results = unittest.TextTestRunner(verbosity=2).run(suite)