test: add common.projectDir

Implement common.projectDir which points to the project directory.

PR-URL: https://github.com/nodejs/node/pull/16208
Fixes: https://github.com/nodejs/build/issues/887


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>