Rollup merge of #100811 - czzrr:master, r=Mark-Simulacrum

Rollup merge of #100811 - czzrr:master, r=Mark-Simulacrum

Fix wrong compiletest filters on Windows

As discussed in [#79334](https://github.com/rust-lang/rust/issues/79334), when calling e.g.
```
python x.py test src/test/ui/expr/compound-assignment/eval-order.rs
```
on Windows, compiletest passes the filter `expr/compound-assignment/eval-order.rs` to libtest, which instead should be `expr\compound-assignment\eval-order.rs`, as that is the file found when collecting tests. This is what I fixed.

I'm not sure how to organize a test for this. Any suggestions?