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 as in !3907 (merged), extensions to support new functionality such as IPv6, and refactorings.

A number of supporting cleanup has been done as well.

I put labels 4G-LTE 5G-NR labels to check that everything still works