How about just don't make it weaker?
These tests are essentially using the phone as a beam...a beam with a very thin web. In I-beam design, the center is the neutral axis and under no stress. An efficient beam will put as much material as possible as far from the neutral axis as possible- it makes an enormous difference, with bending resistance increasing at the square of this distance.
In the following image, the beam on the right, which uses the same amount of material, would have 15x the bending resistance of the beam on the left.
Image
If you made an i-phone size i-beam, going from 6.5mm thick to 8mm increases bending resistance by about 50%...using the SAME amount of material.
That's for an i-beam, but you get the idea. The iphone is more of a U-beam, so instead of having a bottom flange, that load (tension, if you're pushing on the back of the phone) has to be taken by the web itself, pushing the neutral axis VERY close to the top flange (the back of the phone). To make matters worse, a bunch of holes are cut in this web, removing material where it's needed most and creating stress concentrations. These stress concentrations are exactly where the phone fails. It was a servicable design when the phone was thicker, but it causing big problems now. If they want to continue using this design, a good way to strengthen it would be a large rim just behind the screen...perhaps even of a much stronger material bonded to the aluminum, or of a much greater thickness of aluminum than the back plate. This would then take the bending stress instead of the sides of the phone, and the neutral axis (which is under no stress) would move to the centerline of the buttons, where it wouldn't do much harm, instead of the corner of the buttons, where it does the most harm.
All this is just because they keep making the phone thinner without making any other changes to compensate for the reduced bending resistance.