sgsn: Create testcase that verifies that llmes get deleted

On an "unassignment" this code verifies that the LLME will vanish
from the list of LLMEs. We assume that this doesn't create a
memory leak.
diff --git a/openbsc/tests/sgsn/Makefile.am b/openbsc/tests/sgsn/Makefile.am
index 9753acf..af00a34 100644
--- a/openbsc/tests/sgsn/Makefile.am
+++ b/openbsc/tests/sgsn/Makefile.am
@@ -6,5 +6,17 @@
 noinst_PROGRAMS = sgsn_test
 
 sgsn_test_SOURCES = sgsn_test.c
-sgsn_test_LDADD = $(LIBOSMOCORE_LIBS)
+sgsn_test_LDADD = \
+	$(top_builddir)/src/gprs/gprs_llc_parse.o \
+	$(top_builddir)/src/gprs/gprs_llc.o \
+	$(top_builddir)/src/gprs/crc24.o \
+	$(top_builddir)/src/gprs/gprs_sndcp.o \
+	$(top_builddir)/src/gprs/gprs_gmm.o \
+	$(top_builddir)/src/gprs/gprs_sgsn.o \
+	$(top_builddir)/src/gprs/sgsn_vty.o \
+	$(top_builddir)/src/gprs/sgsn_libgtp.o \
+	$(LIBOSMOCORE_LIBS) \
+	$(LIBOSMOGSM_LIBS) \
+	$(LIBOSMOGB_LIBS) \
+	-lgtp