Auto merge of #65630 - ecstatic-morse:graphviz-tidy, r=Mark-Simulacrum

Check all files in `src/test` for `borrowck_graphviz_postflow`

This attribute causes DOT files to be generated in the top-level directory. It is intended to be used only temporarily and should never appear on master. This also tells git to ignore DOT files in the root or the  `mir_dump` directory, which `-Z dump-mir` uses by default.

This will prevent #65071 from occurring again. It needs to be merged after #65629, otherwise `tidy` will start failing.

r? @Mark-Simulacrum