Write RigidBody / Physics Unit Tests

Summary

There are already unit tests that check some basic cloning / copying functionality, but there are none that test actual physics and transformation behavior. Add a few basic tests to ensure physics works properly.

Analysis

• The basic test layout could be to create a scene, make it current and simulate a certain number of frames with a fixed timestep, similar to the sandbox single-frame step.
• Tests could include:
  • A ball falling down onto a platform.
  • Non-static bodies not depending on parent transform, unless AllowParent was set to true.
  • Probably lots of others, including properties, joints, etc., but keep it short and simple for now.