tests: Bluetooth: Add example bsim test

This test is intended to teach the conventions and best practices to a
potential contributor that wants to add a test, for say a new feature.

It is liberally commented on purpose, so new contributors don't have to
spend two weeks understanding the intricacies of the simulator, the bsim
test framework, the native builds, backchannels, etc..

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>