tests: add a C++ Hello World test

This is useful to measure OSv boot speed, IOW, how fast are we without
CLI, Java, etc.

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