tests: initial malloc benchmark

Currently covering three cases: 1 thread allocation, n threads allocation,
and n threads allocations with frees in a different cpu. We should improve
with more cases.

This test was designed to run on Linux as well. Let's keep it this way.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>