Mirrors would work for a low power laser, but not for what they were testing. There are other options though. The simplest one is a spiky elongated nose cone, which would dramatically reduce delivered power density for a missile on head-on trajectory. Add an ablative cone and you can defeat laser power levels realistically available in the near future, compact enough to be placed in a pod under a wing. Bear in mind that chemically powered laser will have limited number of shots available. Electrically charged one (with super-capacitor) will need significant delays between shots. In both cases they can be overwhelmed by a large enough number of missiles.