RenderPipelineValidationTest: Fix vector constructor

Integer literals need to be suffixed with `u` in order to be unsigned. This also includes 0.

Tint has new validation that now checks for this.

Bug: tint:632
Change-Id: I1af07590d93620561df1a0084521eb071d4af7ed
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/44766
Commit-Queue: Ben Clayton <bclayton@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tests/unittests/validation/RenderPipelineValidationTests.cpp

@@ -695,7 +695,7 @@ TEST_F(RenderPipelineValidationTest, FragmentOutputCorrectEntryPoint) {
             return;
         }
         [[stage(fragment)]] fn fragmentUint() -> void {
-            colorUint = vec4<u32>(0, 0, 0, 0);
+            colorUint = vec4<u32>(0u, 0u, 0u, 0u);
             return;
         }
     )");