You should not fail a test of the achilles if your tendon is not ruptured. Get them to test and compare to the other leg to see if it is normal laxity for you. If the results do suggest that the injured leg is ruptured then get to a sports Dr. for an Wikipedia reference-linkMRI. I would treat a partial tear with almost as much care as a conservative rupture. i.e. 6 weeks in the boot before retesting. Let us know how you get on. If it's a partial tear and you begin to work it to early it may go on to a full thickness tear. So be careful at this time.