Add return to simple test

Change-Id: I0432724f3b77912ec547a0700c022f9dbbca7d36
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25460
Reviewed-by: dan sinclair <dsinclair@chromium.org>

test/simple.wgsl

diff --git a/test/simple.wgsl b/test/simple.wgsl
index 92d42ad..d0adf46 100644
--- a/test/simple.wgsl
+++ b/test/simple.wgsl
@@ -15,7 +15,7 @@
 [[location 0]] var<out> gl_FragColor : vec4<f32>;
 
 fn bar() -> void {
-
+  return;
 }
 
 fn main() -> void {