Example and test for entity management edge cases

[Jondolf]

We frequently run into edge cases where spawning/despawning/enabling/disabling different kinds of bodies, colliders, or joints can fail and cause a crash. This is especially common with the persistent contact graph, simulation islands, and collider trees. Entity cloning is particularly problematic and not properly supported right now, in part due to the lack of testing.

We should have some kind of test suite that covers as many of these cases as possible, to make it easier to catch problems and regressions and to debug them.