Auto merge of #12565 - simartin:issue_12564, r=Ms2ger
Auto merge of #12565 - simartin:issue_12564, r=Ms2ger

Issue #12564: Fix "./mach test-tidy --self-test".

Fix https://github.com/servo/servo/issues/12564

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12564
- [X] There are tests for these changes

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