workgroup_size validation: support devices limited to 256 invocations (#4491)

On devices with maxComputeInvocationsPerWorkgroup=256, the
webgpu:shader,validation,shader_io,workgroup_size:workgroup_size:attr="override_expr"
test can fail when requesting a workgroup larger than 256.

Adjust the override expression case to generate no more than 256 invocations.

Author: dneto0

src/webgpu/shader/validation/shader_io/workgroup_size.spec.ts

@@ -162,7 +162,7 @@ const kWorkgroupSizeTests = {
   },
 
   override_expr: {
-    src: `override a = 5;
+    src: `override a = 3;
     override b = 6;
     @workgroup_size(a, b, a + b)`,
     pass: true,