doc: note the system requirements for V8 tests

The `test-v8` Makefile target still requires Python 2,
and it requires a full Xcode install on macOS.

Refs: https://github.com/nodejs/node/pull/36691

PR-URL: https://github.com/nodejs/node/pull/38319


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>