Add Collision System Unit Tests

• [X] Sphere-sphere correctness/robustness.
• [ ] OBB-OBB correctness/robustness.
• [ ] Sphere-OBB correctness/robustness.
• [ ] Collider-collider correctness (colliders defer to the proper concrete type and give the correct result).
• [ ] Grid partitioning correctness (puts spheres and OBBs) in correct grid location.
• [ ] Grid cell iterator correctness.
• [ ] Calculating AABB for colliders (sphere and OBB).