Fix Windows symlink test to give reproducible error messages
Fix Windows symlink test to give reproducible error messages

After a `git clone -c core.symlinks=false` and `cargo test`, the script outputs a Git error as expected.
When running `cargo test` again, the error message changes to "symlink support needs to be enabled".

The test is done for an already existing directory, and the Windows mklink command _fails_ when the symlink exists already.
This commit deletes the link prior to the symlink creation.

dtolnay/cxx,dtolnay/cxx,dtolnay/cxx,dtolnay/cxx