Auto merge of #14051 - birryree:tidy-check-buildbot-steps, r=aneeshusa

Adding linting checks for buildbot_steps.yml

This pull request adds some tidy checks around YAML files, and specifically `buildbot_steps.yml`.

Tidy checks added:

* YAML files are checked for well-formedness/parse-ability
* Whether a YAML file has duplicate keys
* Whether a `buildbot_steps.yml` file contains only mappings to list-of-strings.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [x] These changes fix #13838 (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

…ing checking for correct mappings and duplicate YAML keys. Added unit tests to test_tidy.py.

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---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14051)
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python/tidy/servo_tidy_tests/duplicate_keys_buildbot_steps.yml

@@ -0,0 +1,7 @@
+---
+duplicate_yaml_key:
+    - value1
+other_key:
+    - value2
+duplicate_yaml_key:
+    - value3

python/tidy/servo_tidy_tests/non_list_mapping_buildbot_steps.yml

@@ -0,0 +1,2 @@
+---
+non-list-key: "string string"

python/tidy/servo_tidy_tests/non_string_list_buildbot_steps.yml

@@ -0,0 +1,7 @@
+---
+# This is a buildbot_steps.yml file that should break linting becasue it is not a
+# mapping to a list of strings
+mapping_key:
+    - - list_of_list
+      - sublist_item1
+      - sublist_item2

python/tidy/servo_tidy_tests/test_tidy.py

@@ -177,6 +177,21 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('Unordered key (found b before a)', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
+    def test_yaml_with_duplicate_key(self):
+        errors = tidy.collect_errors_for_files(iterFile('duplicate_keys_buildbot_steps.yml'), [tidy.check_yaml], [], print_text=False)
+        self.assertEqual('Duplicated Key (duplicate_yaml_key)', errors.next()[2])
+        self.assertNoMoreErrors(errors)
+
+    def test_non_list_mapped_buildbot_steps(self):
+        errors = tidy.collect_errors_for_files(iterFile('non_list_mapping_buildbot_steps.yml'), [tidy.check_yaml], [], print_text=False)
+        self.assertEqual("Key 'non-list-key' maps to type 'str', but list expected", errors.next()[2])
+        self.assertNoMoreErrors(errors)
+
+    def test_non_string_list_mapping_buildbot_steps(self):
+        errors = tidy.collect_errors_for_files(iterFile('non_string_list_buildbot_steps.yml'), [tidy.check_yaml], [], print_text=False)
+        self.assertEqual("List mapped to 'mapping_key' contains non-string element", 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"