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/Makefile.am b/src/Makefile.am
index 60fc479..ea5023d 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -52,6 +52,7 @@
 	rand_urandom.c \
 	hlr_vty.c \
 	hlr_vty_subscr.c \
+	gsup_send.c \
 	$(NULL)
 
 osmo_hlr_LDADD = \