Fix 'Dangerous default value [] as argument' issue in tests\test_usage.py

[reactive-firewall]

CodeFactor found an issue: Dangerous default value [] as argument

It's currently on:
tests\test_usage.py:88

Instead the code should be something like:

def timePythonCommand(args=None, stderr=None):
    if args is None:
        args = [None]
   ....