Add test for @@toStringTag on deferred module namespaces (#4360)

Co-authored-by: Linus Groh <[email protected]>

test/language/import/import-defer/deferred-namespace-object/to-string-tag.js

@@ -0,0 +1,35 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-getmodulenamespace
+description: >
+  Deferred namespace objects have a "Deferred Module" @@toStringTag
+info: |
+  ModuleNamespaceCreate ( _module_, _exports_, _phase_ )
+    1. ...
+    1. Let _M_ be MakeBasicObject(_internalSlotsList_).
+    1. ...
+    1. If _phase_ is ~defer~, then
+      1. ...
+      1. Let _toStringTag_ be *"Deferred Module"*.
+    1. Else,
+      1. ...
+    1. Create an own data property of _M_ named %Symbol.toStringTag% whose [[Value]]
+       is _toStringTag_ and whose [[Writable]], [[Enumerable]], and [[Configurable]]
+       attributes are false.
+    1. Return _M_.
+
+flags: [module]
+features: [import-defer]
+includes: [propertyHelper.js]
+---*/
+
+import defer * as ns from "./dep_FIXTURE.js";
+
+verifyProperty(ns, Symbol.toStringTag, {
+  value: "Deferred Module",
+  writable: false,
+  enumerable: false,
+  configurable: false,
+});