test: actually test tty `getColorDepth()`

TTY tests should almost never be placed in `/parallel/`. Skipping TTY
tests there due to missing tty fds just means they will never be run,
ever, on any system.

This moves the tty-get-color-depth test to `/pseudo-tty/` where the test
runner will actually make a pty fd.

Refs: https://github.com/nodejs/node/pull/17615
PR-URL: https://github.com/nodejs/node/pull/18800


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>