I'm not sure exactly what is stopping you here, but I'd suggest another organization of the job.

Let your web-app create a file. Let a cron (or at) -job start every, say, 20 second to see if this file exist. If it exist, read it in and remove it. The file contains instructions on what to test. The cronjob now runs the tests and make the results available for the admin app to pick up and present to the user.