add gsup_test to catch OS#3231

Encode an Insert Subscr Data with is_ps == true to trigger the encoding bug
described in OS#3231, i.e. show that it is fixed.

Move osmo_gsup_addr_send() to a separate .c file, so that it can be overridden
in the regression test to just dump the msgb instead.

I used this test to reproduce issue OS#3231, and now that it's here we might as
well keep it, and possibly expand on it in the future.

Related: OS#3231
Change-Id: Id1453351758f3e1a9ff03bd99fefaf51886e77da
diff --git a/src/luop.h b/src/luop.h
index 053a025..d690243 100644
--- a/src/luop.h
+++ b/src/luop.h
@@ -28,6 +28,7 @@
 #include <osmocom/gsm/gsup.h>
 
 #include "db.h"
+#include "gsup_server.h"
 
 #define CANCEL_TIMEOUT_SECS	30
 #define ISD_TIMEOUT_SECS	30