Bluetooth: Mesh: Add bsim tests for transport

Adds a test suite for Bluetooth Mesh in bsim_bt tests, including 10
tests for common transport behavior. This test suite includes a readme
and a simple framework for adding more mesh tests in the future.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>