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 an empty index page before starting the first test that
nofifies users that the test is running, but no results are available
yet. 'report' replaces it with the usual results page as soon as the
first test finished.

The empty index page is generated after runner checked that the test is
valid, but before 'runner.pre_run' is executed.