Change terminology of image diff to differentiate from pixel tests.

Change-Id: I957199b63f6049a586b8de2688501f5ae3449964
Reviewed-on: https://pdfium-review.googlesource.com/7038
Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
diff --git a/testing/tools/test_runner.py b/testing/tools/test_runner.py
index 81225b7..8f0670d 100644
--- a/testing/tools/test_runner.py
+++ b/testing/tools/test_runner.py
@@ -83,7 +83,7 @@
         return False, results
     else:
       if (self.enforce_expected_images
-          and not self.test_suppressor.IsPixelDiffSuppressed(input_filename)):
+          and not self.test_suppressor.IsImageDiffSuppressed(input_filename)):
         print 'FAILURE: %s; Missing expected images' % input_filename
         return False, results