I think that I'd have one page with a button on it to initiate the process in the background, and another page to take you to the output of the last run.

When you click the button, update the text for the output page to something like "Currently running, but here's the output of the previous run:", and disable the button. Then have a cron jobscheduled task periodically run and check the timestamp of that file. If it's new, start the test! When the test finishes, it simply resets the output page to the normal test output.

Just a thought...

...roboticus