[test] Slim down test that exceeds stack limit

BUG=v8:4928
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1903043006

Cr-Commit-Position: refs/heads/master@{#35688}

test/webkit/fast/js/excessive-comma-usage.js

@@ -34,7 +34,7 @@ for (var i = 0; i < 5000; i++)
 declarationTestString += ";return true;";
 
 var commaExpressionTestString = "1";
-for (var i = 0; i < 5000; i++)
+for (var i = 0; i < 2500; i++)
     commaExpressionTestString += ",1";
 commaExpressionTestString += ";return true;";