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/tests/Makefile.am b/tests/Makefile.am
index f1cc710..58679ed 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -2,6 +2,7 @@
 	auc \
 	gsup_server \
 	db \
+	gsup \
 	$(NULL)
 
 # The `:;' works around a Bash 3.2 bug when the output is not writeable.