tests/vty: Add VTY tests to the osmo-stp

There seems to be issues with the write handling of the osmo-stp
and so far we did not enable vty tests here. Make it possible to
enable the tests, fix the VTY strings, change string to OsmoSTP.

Change-Id: I547fd4840d86ce16e8589fb63802dd7099781194
9 files changed