Display the default values as part of the test_runner.py help

Summary:
This diffs change the default help renderer to display the default value
for each parameter. This fixes the wrong hardcoded default value
displayed for the number of jobs.

Test Plan:

test_runner.py --help

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4374