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/i460_mux/i460_mux_test.c b/tests/i460_mux/i460_mux_test.c
index 9d5fcf7..7695cb4 100644
--- a/tests/i460_mux/i460_mux_test.c
+++ b/tests/i460_mux/i460_mux_test.c
@@ -395,4 +395,5 @@
 	test_16k_subchan();
 	test_8k_subchan();
 	test_unused_subchan();
+	return 0;
 }