[qa] make extended cutoff configurable

Summary:
Add a 'cutoff' flag to test_runner.py that is an integer number of seconds.
The cutoff flag controls which tests run by how long they take in seconds.

Test Plan:

./test/functional/test_runner.py --cutoff=6000

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1929