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/configure.ac b/configure.ac
index a632a1d..cedba45 100644
--- a/configure.ac
+++ b/configure.ac
@@ -104,5 +104,6 @@
 	tests/auc/Makefile
 	tests/auc/gen_ts_55_205_test_sets/Makefile
 	tests/gsup_server/Makefile
+	tests/gsup/Makefile
 	tests/db/Makefile
 	)