[qa] bump test timeout when running on the build system

Summary:
Currently, the tests time out after 20 minutes. Some of the
extended tests seem to take longer than this on our current
VMs and time out.

Test Plan:
Allow CI to run the test suite, and also run:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, movrcx

Reviewed By: movrcx

Subscribers: movrcx, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1428