Auto merge of servo#10758 - jdm:tidy_check_completeness, r=Wafflespeanut

Ensure that we aren't missing any errors in the tidy self-test.

r? @wafflespeanut

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/10758)
<!-- Reviewable:end -->