extra/runner: Do not show "404" while the first test is running

It's quite misleading that there is no index.html in the test results
directory until the first test is completed; users new to `runner`
might think that they entered the wrong address or an error occurred
when they are trying to open the test results.

Generate a row in the table of the index page while a test is being
executed. It has white background with grey text, and the exit status
is "still running".

This is done by skipping `report` if it's run with no arguments or
--classify-only and no exit-status is found in the test run directory,
but executing it if it's run with --html-only.
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().toUTCTime()` 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.
runner/templates/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.