tools: make test.py Python 3 compatible

PR-URL: https://github.com/nodejs/node/pull/25767


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: cclauss <cclauss@me.com>
Reviewed-By: gengjiawen <technicalcute@gmail.com>