enhancement: hide progress bar for CI

[manzt]

Thanks for the awesome work on web-test-runner! We recently migrated from karma and it's been a joy to use.

Is it possible to hide the progress bar from printing? I found the --static-logging flag but this does not seem to have the desired effect. See the cluttered logs in GH Actions: https://github.com/higlass/higlass/runs/8121174917?check_suite_focus=true