test: improve tty.getColorDepth coverage

PR-URL: https://github.com/nodejs/node/pull/19446


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>