Canonicalize path before filtering (#19407)
## Summary
This came up on
[Discord](https://discord.com/channels/1039017663004942429/1343692072921731082/1395447082520678440)
and also in #19387, but on macOS the tmp directory is a symlink to
`/private/tmp`, which breaks this filter. I'm still not quite sure why
only these tests are affected when we use the `tempdir_filter`
elsewhere, but hopefully this fixes the immediate issue. Just
`tempdir.path().canonicalize()` also worked, but I used `dunce` since
that's what I saw in other tests (I guess it's not _just_ these tests).
Some related links from uv:
-
https://github.com/astral-sh/uv/blob/1b2f212e8b2f91069b858cb7f5905589c9d15add/crates/uv/tests/it/common/mod.rs#L1161-L1178
-
https://github.com/astral-sh/uv/blob/1b2f212e8b2f91069b858cb7f5905589c9d15add/crates/uv/tests/it/common/mod.rs#L424-L438
- https://github.com/astral-sh/uv/pull/14290
Thanks to @zanieb for those!
## Test Plan
I tested the `main` branch on my MacBook and reproduced the test
failure, then confirmed that the tests pass after the change. Now to
make sure it passes on Windows, which caused most of the trouble in the
first PR!
