Fix wrong compiletest filters on Windows

As discussed in [rust-lang#79334](rust-lang#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?

Fix wrong compiletest filters on Windows

As discussed in [rust-lang#79334](rust-lang#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?

Fix wrong compiletest filters on Windows

As discussed in [rust-lang#79334](rust-lang#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?

Fix wrong compiletest filters on Windows

As discussed in [rust-lang#79334](rust-lang#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?

Rollup of 11 pull requests

Successful merges:

 - rust-lang#96240 (Stabilize `const_ptr_offset_from`.)
 - rust-lang#99784 (Make forward compatibility lint deprecated_cfg_attr_crate_type_name deny by default)
 - rust-lang#100811 (Fix wrong compiletest filters on Windows)
 - rust-lang#100924 (Smaller improvements of tidy and the unicode generator)
 - rust-lang#100953 (Update documentation for `write!` and `writeln!`)
 - rust-lang#101018 (rustdoc: omit start/end tags for empty item description blocks)
 - rust-lang#101044 (rustdoc: remove unused CSS for `hidden-by-*-hider`)
 - rust-lang#101046 (rustdoc: remove incorrect CSS selector `.impl-items table td`)
 - rust-lang#101057 (Merge implementations of HIR fn_decl and fn_sig.)
 - rust-lang#101062 (rustdoc: remove empty extern_crates and type="text/javascript" on script)
 - rust-lang#101063 (Merge duplicated CSS rules)

Failed merges:

 - rust-lang#101055 (Use smaller span for suggestions)

r? `@ghost`
`@rustbot` modify labels: rollup