stp: Add tests for setups with multiple "routing context 0" ASPs

These two test cases verify that
* libosmo-sigtran doesn't include a routing context IE even in ASP
  role when using a ASP with a single routing context 0 configured
* osmo-stp can route between a single SG-role ASP with routing context
  to two ASP-role ASPs which both have routing context 0 (i.e. no
  routing context IE in use)

The tests do not pass with current libosmo-sigtran until OS#6003 is
fixed, presumably by Change-Id I5ce89b393a3b950ab7fd1eace9038718c9efcc51

Change-Id: I81052ece7d1cc8b43da6155356ed1c4d9620acdc
Related: OS#6003
2 files changed