stp: fix TC_rkm_unreg_registered

Create the routing key dynamically, before trying to unregister it. The
previous version tried to unregister a statically configured routing
key, as in TC_rkm_unreg_never_registered, but expected success instead
of the proper ERR_NOT_REG.

So after fixing osmo-stp to make TC_rkm_unreg_never_registered pass,
this test failed.

Related: OS#4239
Change-Id: I7d2f9eb298778a8e60c7e73f314bc73528e85406
1 file changed