Add a simple GTP unit test
Add a simple unit test for GTP that as of now tests the main
functionality, i.e., for a single UE, set up a bearer between two
endpoints and checking that traffic sending works. This is a start to
extend it further, e.g., with API changes, extensions to support new
functionality such as IPv6, and refactorings.
A number of supporting cleanup has been done as well.
Robert Schmidt
8abb535481