trans_free: tear down conn when last transaction is done

In trans_free(), call subscr_conn_release_when_unused(), so that we are sure to
clean up after the last transaction is done.

This fixes an error where a conn lingered after a CC failure, because that code
path forgot to trigger cleanup.

Rationale: so far we were triggering the release check after each DTAP dispatch
(compl_l3 and "normal" DTAP), which is sufficient for properly closed
transactions. We also need a check for when a timeout clears an erratic trans.

Adjust test expectation of test_call_mo_to_unknown_timeout to show that the
error is now fixed.

msc_vlr_test_reject_concurrency now sees an additional release checking event
when the SMS transaction is done, which is expected and does not affect the
test otherwise.

Related: OS#2779
Change-Id: I46ff2e9b09b67e4e0d79cccf8c04936f17281fcb
diff --git a/src/libmsc/osmo_msc.c b/src/libmsc/osmo_msc.c
index 01e44f3..d0c5235 100644
--- a/src/libmsc/osmo_msc.c
+++ b/src/libmsc/osmo_msc.c
@@ -47,7 +47,7 @@
 		gsm411_sapi_n_reject(conn);
 }
 
-static void subscr_conn_release_when_unused(struct gsm_subscriber_connection *conn)
+void subscr_conn_release_when_unused(struct gsm_subscriber_connection *conn)
 {
 	if (!conn)
 		return;