Snapshot tests: canonicalize with dunce (#5582)
This uses dunce [0] to consistently canonicalize paths used in
Turbopack's tests. Previously, `REPO_ROOT` used Windows UNC paths while
the test path in snapshot tests did not. This caused an issue when using
`string.strip_prefix()` to determine relative paths.
[0] https://crates.io/crates/dunce
