extra/runner/index.html: Update duration of the test still running

Update the "Duration" value of the test still running when the page
is loaded with the time elapsed since the test started. Start time of
the test is read from "Timestamp".

Implementation issues:
- `Date.parse` and `Date.now()` return Unix timestamps but
  `Date().toString()` prints time in the current time zone.
  `Date().toUTCString()` must be used to get the right duration.
- I considered auto-increasing the duration value using `setInterval`
  but it would be misleading. The page is not refreshed periodically,
  therefore the timer would still be running after the test actually
  finished.
extra/runner/index.html: Do not show index page of the test still running

As that page doesn't have any value with tests still running, it's
better to make the row of the ongoing test non-clickable.