I was having limited success fuzzing locally because apparently the
fuzzer was spawning too many threads. Looking into it that indeed
appears to be the case! The threads which time out runtime of wasm only
exit after the sleep has completely finished, meaning that if we execute
a ton of wasm that exits quickly each run will generate a sleeping thread.
This commit fixes the issue by using some synchronization to ensure the
sleeping thread exits when our fuzzed run also exits.
e09b9400f8