tests/*: fix control reaches end of non-void func

Prepare to set -std=gnu89 in a future commit, which will cause gcc warn
about "control reaches end of non-void function" in main().

Change-Id: I7c33cac30e5859060f083813d8433011f5eaf0d0
diff --git a/tests/it_q/it_q_test.c b/tests/it_q/it_q_test.c
index 0d75452..28e32d8 100644
--- a/tests/it_q/it_q_test.c
+++ b/tests/it_q/it_q_test.c
@@ -116,4 +116,5 @@
 	tc_alloc();
 	tc_queue_length();
 	tc_eventfd();
+	return 0;
 }