test: fix test runner for Python 3 on Windows

Explicitly open files with utf8 encoding, otherwise the system could use
another encoding such as latin1 by default.

PR-URL: https://github.com/nodejs/node/pull/30023


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>