test: call - add call on-hold/resume test (#990)

* sess: add sipsess_ack_pending()

can be used for unit tests

* sess: add NULL pointer check

* sess: add doxygen to sipsess_ack_pending()

include/re_sipsess.h

@@ -73,3 +73,4 @@ bool sipsess_refresh_allowed(const struct sipsess *sess);
 void sipsess_close_all(struct sipsess_sock *sock);
 struct sip_dialog *sipsess_dialog(const struct sipsess *sess);
 void sipsess_abort(struct sipsess *sess);
+bool sipsess_ack_pending(const struct sipsess *sess);

src/sipsess/sess.c

@@ -352,3 +352,17 @@ bool sipsess_refresh_allowed(const struct sipsess *sess)
 	return ((sess->established || sess->refresh_allowed)
 		&& !sess->terminated && !sess->awaiting_answer);
 }
+
+
+/**
+ * Return true if there is an open SIP Session Reply for which an ACK is
+ * expected
+ *
+ * @param sess      SIP Session
+ *
+ * @return True if ACK is pending, otherwise false
+ */
+bool sipsess_ack_pending(const struct sipsess *sess)
+{
+	return sess && sess->replyl.head ? true : false;
+}