The check's tests row lands from pull #2578's branch, right on a phone and for a screen reader #2584

Replaces #2383. Pull #2578 is stuck after three change requests. Start from its branch scout/2383-a-check-s-tests-row-shows-a-progress-bar rebased on main, and open a new pull.

Done looks like:

• [ ] The live region (role=status, aria-live) wraps only the rows and the words line, not the pins.
• [ ] The slot reserves its height only while the run is open, and is capped with max-height after.
• [ ] With pins, the slot has tabindex="0" and a label.
• [ ] The bar has aria-label="Test files started" and is indeterminate (no value) when the count is unknown.
• [ ] One render-level test of runView asserts these. Another asserts that the served script refetches on (fail) lines. One more covers the terminal line.