Work around a race condition where multiple AFL instances bind to the same CPU

[anon8675309]

This is the same issue @genewitch found and I can confirm it is also a problem on 64-core machines as well. The issue is that multiple instances of AFL will bind to the the same core. To see this in action: head -n 25 $OUTPUT_DIR/$TARGET/*/afl-launch.log | grep binding | sort

I also did some experimentation and a 100 ms delay still resulted in two threads binding to CPU #0, but a 250 ms delay seems to reliably avoid any CPU contention.