the physics engines to return the name that is specified in the INI
file ("physics = XXX") as the type of engine.
This os function is a little different than the others in that it
does not throw an exception of one is not privilaged to use it.
It merely returns an empty string.
This was an oversight when removing some race conditions from PhysicsActor setting recently.
Regression tests extended to probe this code path.
Extending regression tests required implementation of a BasicPhysicsPrim (there was none before). However, BasicPhysics plugin is still of no current practical use other than to fill in as a component for other parts of regression testing.
