test: add --test-root option to test.py

This way we can specify a custom path for the test folder, e.g.
when building addons separately from the source tree.

PR-URL: https://github.com/nodejs/node/pull/26093


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>