Print filenames in the diff

tests/pretty_format_json_test.py

@@ -1,3 +1,4 @@
+import os
 import shutil
 
 import pytest
@@ -110,9 +111,13 @@ def test_badfile_main():
 def test_diffing_output(capsys):
     resource_path = get_resource_path('not_pretty_formatted_json.json')
     expected_retval = 1
+    a = os.path.join('a', resource_path)
+    b = os.path.join('b', resource_path)
     expected_out = '''\
---- \n+++ \n@@ -1,6 +1,9 @@
- {
+--- {}
++++ {}
+@@ -1,6 +1,9 @@
+ {{
 -    "foo":
 -    "bar",
 -        "alist": [2, 34, 234],
@@ -124,9 +129,9 @@ def test_diffing_output(capsys):
 +  ],
 +  "blah": null,
 +  "foo": "bar"
- }
+ }}
 
-'''
+'''.format(a, b)
     expected_err = 'File {} is not pretty-formatted\n'.format(resource_path)
 
     actual_retval = main([resource_path])