tests: add benchmark for calibrating setpriority()

Our setpriority() implementation arbitrarily decided to make nice
value -20 mean that such a thread will get 5 times more runtime than
a competing thread with default nice value (0).

We never verified whether Linux actually uses this constant 5, or
indeed whether it is a logarithmic unit, as we assumed (i.e., adding
1 to the nice value, *multiplies* the runtime ratio by some constant).

The benchmark in this patch, which can run on both Linux and OSv,
measures the effect of setpriority() on ratio of thread runtime.

Running this on OSv confirms the behavior we tried to get (nice value
-20 gets 5 times the runtime). Running this on Linux indeed shows
logarithmic behavior, but nice -20 doesn't correspond to just 5-fold
increase of runtime, but actually a 86-fold increase.

This benchmark is related to issue #438.

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