stp: Introduce STP_Tests*.ttcn for testing OsmoSTP

In the past, we were automatically running [large parts of] the nplab
M3UA and SUA test suites, but those implement only a fraction of the
functionality.  Particularly, they don't cover the non-standard IPA
behavior, and they don't cover RKM (routing key management).

Let's introduce an initial set of STP tests with this patch.  We try
to not duplicate nplab here, and implement bits not covered there.

Change-Id: I628a87385cac0dfe708a0d74a5088fbd5a4790cd
diff --git a/Makefile b/Makefile
index 99243d6..4ceacf3 100644
--- a/Makefile
+++ b/Makefile
@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-SUBDIRS=bsc bsc-nat bts ggsn_tests hlr mgw msc pcu remsim sccp selftest sgsn sip sysinfo
+SUBDIRS=bsc bsc-nat bts ggsn_tests hlr mgw msc pcu remsim sccp selftest sgsn sip stp sysinfo
 
 NPROC=$(shell nproc 2>/dev/null)
 ifeq ($(NPROC),)