The main motivation is to move toward running those tests for a specific
target, that is, if a test won't run on Windows, then we shouldn't build
it. This was previously the default behavior and prevented a successful
run on AppVeyor.
I borrowed this pattern from the tests in the Cargo project.
This commit adds the `ScopedFile` type, which wraps and derefs to a `File`. When
a `ScopedFile` is dropped, it removes the underlying file from the
filesystem. This is useful for temporary, generated files in tests.
Due to canonicalize()'s use of GetFinalPathNameByHandleW() on Windows,
the resolved path starts with '\\?\' to extend the limit of a given path
to 32,767 wide characters.
To address this issue, we remove this prepended string if available.
