Add tests and enhance the upf test suite to be closer to real world
usage:
- properly verify the F-TEIDs chosen by osmo-upf.
- add tests with two-step session creation, i.e. with a Session
Establishment followed by Session Modification indicating the remote
F-TEID to use on the core side, as is the usual case.
- Add module parameters for network instances to use in the test;
dynamically configure osmo-upf's "netinst" config via VTY.
- pass Network Instance in Create PDR, verify that osmo-upf returns the
matching GTP IP addresses in Created PDR.
Related: SYS#6192 SYS#5599
Change-Id: I440466f1cc9689391869ac2579a4497ef6008adb
Neels Hofmeyr