test: specify 'dir' for directory symlinks

Directory symlinks in Windows require the 'dir' flag to be passed to
create the symlink correctly.

PR-URL: https://github.com/nodejs/node/pull/19049


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>