Rename file causing `check-json` to fail

- `mv duplicated_key_json.{json,notjson}`
- change reference to the file in `tests/check_json_test.py`

Fixes #600

tests/check_json_test.py

@@ -9,7 +9,7 @@ from testing.util import get_resource_path
         ('bad_json.notjson', 1),
         ('bad_json_latin1.nonjson', 1),
         ('ok_json.json', 0),
-        ('duplicate_key_json.json', 1),
+        ('duplicate_key_json.notjson', 1),
     ),
 )
 def test_main(capsys, filename, expected_retval):