test: fix test-tty-get-color-depth

If getTTYfd returns 0, and stdin is not writable (like on Windows),
trying to create WriteStream will fail. This commit fixes that by
skipping fd 0.

PR-URL: https://github.com/nodejs/node/pull/18478


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>