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.
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<!-- 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
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…ing checking for correct mappings and duplicate YAML keys. Added unit tests to test_tidy.py.
Source-Repo: https://github.com/servo/servo
Source-Revision: 21ad1c210997daba82ec49e1572c7b0634b6f337
1bc5a21936