extra/runner summary report: Improve confidence interval display

Instead of:

    Passed: 74/80 (85% - 97%). Failed: 6/80 (3.4% - 17%). Errors: 0/80 (0% - 4.6%).

We now show:

    Passed: 74/80 (91±6%). Failed: 6/80 (9.4±6%). Errors: 0/80 (<4.6%).

Which is much clearer.