Canonicalize paths when running tests (#23655)

In the Windows test environment, the paths generated by `temp_tree()`
are symlink paths, which causes certain tests to fail.

I later noticed that when opening a project, we seem to always use
`canonicalize` to normalize the paths, as shown here:
https://github.com/zed-industries/zed/pull/21039.

This PR adopts a similar approach for the test environment to address
the issue.

Release Notes:

- N/A