Improve simulator shutdown behavior for regression tests

[sgherbst]

If a simulator binary doesn't exit on its own, it is terminated via atexit. However, this isn't quite enough when running multiple regression tests from a single Python instance, since multiple unused simulator binaries can stack up, slowing things down. Perhaps simulations could be shut down automatically when an SbDut object is deleted. Will have to consider whether there are cases when we want the simulation to keep running even though the SbDut object that started it is gone.