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

tests/i460_mux/i460_mux_test.c

@@ -395,4 +395,5 @@ int main(int argc, char **argv)
 	test_16k_subchan();
 	test_8k_subchan();
 	test_unused_subchan();
+	return 0;
 }

tests/it_q/it_q_test.c

@@ -116,4 +116,5 @@ int main(int argc, char **argv)
 	tc_alloc();
 	tc_queue_length();
 	tc_eventfd();
+	return 0;
 }