Auto merge of #12317 - tallowen:add-tidy-test, r=jdm

[tidy] Test json parsing functionality

We didn't have a test for json parsing so lets start with that

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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python/tidy/servo_tidy_tests/malformed_json.json

@@ -0,0 +1,4 @@
+{
+    "key": "value",
+    "other_key": "value_with_bad_quote'
+}

python/tidy/servo_tidy_tests/test_tidy.py

@@ -104,6 +104,11 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('emacs file variables present', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
+    def test_malformed_json(self):
+        errors = tidy.collect_errors_for_files(iterFile('malformed_json.json'), [tidy.check_json], [], print_text=False)
+        self.assertEqual('Invalid control character at: line 3 column 40 (char 61)', errors.next()[2])
+        self.assertNoMoreErrors(errors)
+
     def test_lock(self):
         errors = tidy.collect_errors_for_files(iterFile('duplicated_package.lock'), [tidy.check_lock], [], print_text=False)
         msg = """duplicate versions for package "test"