tests/gb: Show invoked signals in test output

Register an osmo signal handler to print a short notice about every
SS_L_NS signal that is generated while processing the tests.

Sponsored-by: On-Waves ehf
diff --git a/include/osmocom/gprs/gprs_ns.h b/include/osmocom/gprs/gprs_ns.h
index c709312..d16068b 100644
--- a/include/osmocom/gprs/gprs_ns.h
+++ b/include/osmocom/gprs/gprs_ns.h
@@ -176,6 +176,9 @@
 /* Add NS-specific VTY stuff */
 int gprs_ns_vty_init(struct gprs_ns_inst *nsi);
 
+/* Resturn peer info as string (NOTE: the buffer is allocated statically) */
+const char *gprs_ns_format_peer(struct gprs_nsvc *nsvc);
+
 #define NS_ALLOC_SIZE	2048
 #define NS_ALLOC_HEADROOM 20
 static inline struct msgb *gprs_ns_msgb_alloc(void)