Don't test namespaces in idlharness if not exposed

Instead, if a namespace is not exposed, test that there is no global
property of that name, similar to what we do with interfaces.

resources/idlharness.js

@@ -3451,6 +3451,17 @@ IdlNamespace.prototype.test_self = function ()
 
 IdlNamespace.prototype.test = function ()
 {
+    // If the namespace object is not exposed, only test that. Members can't be
+    // tested either
+    if (!this.exposed) {
+        if (!this.untested) {
+            subsetTestByKey(this.name, test, function() {
+                assert_false(this.name in self);
+            }.bind(this), this.name + " namespace: nonexistence of namespace object");
+        }
+        return;
+    }
+
     if (!this.untested) {
         this.test_self();
     }