A Volume class to represent arbitrary rectangles has been added.

It is like an SDL_Rect but without that dependency on the SDL.
Volume.h now includes "common/vector.h"
There is now a function that can check whether two Volumes are intersecting.

Tests have also been implemented for this function and the other in
src/server/collision_util.h (pong::isIn).
Removed unused test files.
Added LocalServer tests.