Fix tests involving temp directory on macOS (#2052)
* Fixed typos
* Always resolve the path to a temp file
The problem is that the code trying to create a symlink in
`_create_symlink` tries to find a relative path from src to dst in
```
relative_src = os.path.relpath(abs_src, abs_dst_folder)
```
The problem is that on macOS, abs_src and abs_dst have a different root
path due to the way the OS handles temp directories.
The one from the fixture is a resolved path to `/private/var/folders/…`
while the one from `SoftTemporaryDirectory` is still an unresolved path
pointing to `/var/folders/` (`/var/` is actually `var -> private/var`).
And thus, their common ancestor is `/` which we try use to write the
symlink to (which obviously fails).
Removed resolving the path from the fixture and instead resolve the path
all the time in `SoftTemporaryDirectory` (which is used by the fixture)
so all call sites get the same resolved path.
* Fix usages of SoftTemporaryDirectory
* Fix import
