Automate running of tests of linters

Description

The unit test files in `scripts/xsslint/tests` (and possibly alongside) appear not to be running; I mentioned this at Tools Team office hours and we agreed they probably should be running automatically on PRs.

I know because these changes weren't necessary to get the corresponding PR's status to green.

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User Impact Summary

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Done

Assignee

Mike Youngstrom

Reporter

Matthew Hughes

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