[test_runner] Fix unidash option for junit output to be one character

Summary: > -ju is absolutely non standard. unidashed parameter are uni letters, contrary to double dashed ones.

Test Plan: Harbormaster build will be green

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3692