test: Add test cases for zero value constructors

Fixed: tint:477
Change-Id: I087c24904ab2f38524ab2c39092fbfb277fdba3b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/55256
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/test/expressions/zero_init/vec4/u32.wgsl.expected.msl b/test/expressions/zero_init/vec4/u32.wgsl.expected.msl
new file mode 100644
index 0000000..9d55361
--- /dev/null
+++ b/test/expressions/zero_init/vec4/u32.wgsl.expected.msl
@@ -0,0 +1,7 @@
+#include <metal_stdlib>
+
+using namespace metal;
+void f() {
+  uint4 v = uint4();
+}
+