Employ the "swarm testing" idea in spirv-fuzz (#2890)

This change to spirv-fuzz uses ideas from "Swarm Testing" (Groce et al. 2012), so that a random subset of fuzzer passes are enabled. These passes are then applied repeatedly in a randomized fashion, with the aggression with which they are applied being randomly chosen per pass.

There is plenty of scope for refining the probabilities introduce in this change; this is just meant to be a reasonable first effort.