Add test for "fetch redirect", add CSS value validation for external render (#37207)

By the way, fix the checkAppUrl message for #37212

web_src/js/globals.d.ts

@@ -68,6 +68,15 @@ interface Window {
   turnstile: any,
   hcaptcha: any,
 
+  // Make IIFE private functions can be tested in unit tests, without exposing the IIFE module to global scope.
+  // Otherwise, when using "export" in IIFE code, the compiled JS will inject global "var externalRenderHelper = ..."
+  // which is not expected and may cause conflicts with other modules.
+  testModules: {
+    externalRenderHelper?: {
+      isValidCssColor(s: string | null): boolean,
+    }
+  }
+
   // do not add more properties here unless it is a must
 }