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.

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- [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).

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- [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.

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