[WGSL] Fix incorrect for-loop test

https://bugs.webkit.org/show_bug.cgi?id=261758
rdar://115735593

Reviewed by Dan Glastonbury.

This is a small follow-up from 268078@main: I fixed the for-loop scoping, but
missed that a test that was relying on the incorrect behavior started failing.

* Source/WebGPU/WGSL/tests/valid/for.wgsl:

Canonical link: https://commits.webkit.org/268175@main

Source/WebGPU/WGSL/tests/valid/for.wgsl

@@ -6,6 +6,7 @@ fn testForStatement() {
     for (var i = -1; i <= 1; i++) {
     }
 
+    var i = 0;
     for (i++; i <= 0; i--) {
     }
 }