[test_runner] Fix junitoutput typo

Summary: See title.

Test Plan:

test_runner.py -ju junit_results2.xml
test_runner.py --junitoutput junit_results3.xml

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3690