tests: more load-balancer tests

This patch adds two more load-balancing tests to tests/misc-loadbalance.cc:

1. Three threads on two cpus. If load-balancing is working correctly, this
   should slow down all threads x1.5 equally, and not get two x2 threads
   and one x1.

   Our performance on this test are fairly close to the expected.

2. Three threads on two cpus, but one thread has priority 0.5, meaning it
   should get twice the CPU time of the two other threads, so fair load
   balancing is to keep the priority-0.5 thread on its own CPU, and the
   two normal-priority threads together on the second CPU - so at the end
   the priority-0.5 thread will get twice the CPU time of the other threads.

   Unfortunately, this test now gets bad results (x0.93,x0.94,x1.14
   instead of x1,x1,x1), because our load balancer currently doesn't take
   into account thread priorities: It thinks the CPU running the
   priority-0.5 thread has load 1, while it should be considered to have
   the load 2.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>