[test_runner] Use the daemon property directly instead of setDaemon()

Summary: setDaemon() is deprecated: https://docs.python.org/3/library/threading.html#threading.Thread.setDaemon

Test Plan:

test_runner.py

Ctrl+C can still be used to kill test_runner.py as expected.

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D5583