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

(SkipBuildbotRuns)
NOTREECHECKS=True
NOTRY=True
R=rmistry@google.com
TBR=rmistry

Author: epoger@google.com

Review URL: https://codereview.chromium.org/252583005

git-svn-id: http://skia.googlecode.com/svn/trunk@14385 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/rebaseline_server/base_unittest.py b/gm/rebaseline_server/base_unittest.py
index 172a8c0..eec0c8d 100755
--- a/gm/rebaseline_server/base_unittest.py
+++ b/gm/rebaseline_server/base_unittest.py
@@ -49,8 +49,11 @@
       # 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."""