[popcornmix]

but I'm not sure if there's anything like that available for the RPi.

On Raspbian you can install the stress package which can help test the CPU and memory. Testing the GPU overclock is trickier, Quake used to be a good test for that, maybe running omxplayer at the Raspbian command line could give an indication of stability.

Video decode doesn't really stress gpu. Quake or hello_triangle2(*) are better.

(*) if force_turbo is not set then run something on arm at the same time (e.g. stress) to get arm busy enough to switch to turbo.