Implement expected stderr output checks and out-of-tree single test execution

Summary:
The abc-cmdline.py test illustrates how tests can check their expected stderr traces.

Refer also to the OutputChecker class in qa/rpc-tests/test_framework/outputchecker.py.

Test Plan:

make check
qa/pull-tester/rpc-tests.py abc-cmdline.py

Reviewers: #bitcoin_abc, sickpig, kyuupichan, awemany, freetrader

Reviewed By: #bitcoin_abc, freetrader

Differential Revision: https://reviews.bitcoinabc.org/D230