ctrl: Introduce cmd SET subscriber.delete <imsi>

On success SET_REPLY contains the ID of the deleted subscriber.

Related: SYS#5993
Change-Id: I24c6149d7c223a12549d712a7929666220c0210f
diff --git a/tests/test_subscriber.ctrl b/tests/test_subscriber.ctrl
index 5b49702..889f0a7 100644
--- a/tests/test_subscriber.ctrl
+++ b/tests/test_subscriber.ctrl
@@ -670,3 +670,6 @@
 periodic_lu_timer	0
 periodic_rau_tau_timer	0
 lmsi	00000000
+
+SET 112 subscriber.delete 901991234567891
+SET_REPLY 112 subscriber.delete 124