[qa] make extended cutoff configurable
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.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: deadalnix, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1929