gbproxy: Add VTY command to override the node selection function

This is mainly useful when testing SGSN pooling

Change-Id: I2894320413dbd0b9aec9a9bc9b88ce7cdeed206b
Related: SYS#5115, OS#4472
diff --git a/include/osmocom/sgsn/gb_proxy.h b/include/osmocom/sgsn/gb_proxy.h
index dd45c2f..ad5bb27 100644
--- a/include/osmocom/sgsn/gb_proxy.h
+++ b/include/osmocom/sgsn/gb_proxy.h
@@ -60,6 +60,10 @@
 		/* NRI bitlen and usable NULL-NRI ranges */
 		uint8_t nri_bitlen;
 		struct osmo_nri_ranges *null_nri_ranges;
+
+		/* Used for testing: If not NULL then this SGSN is returned by
+		 * gbproxy_sgsn_by_tlli() */
+		struct gbproxy_sgsn *nsf_override;
 	} pool;
 
 	/* hash table of all BSS side Gb peers */