Add blank XRTest interface

components/script/dom/webidls/XRTest.webidl

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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://github.com/immersive-web/webxr-test-api/
+
+[Exposed=Window, Pref="dom.webxr.test"]
+interface XRTest {
+  // // Simulates connecting a device to the system.
+  // // Used to instantiate a fake device for use in tests.
+  // Promise<FakeXRDeviceController> simulateDeviceConnection(FakeXRDeviceInit);
+
+  // // Simulates a device being disconnected from the system.
+  // Promise<void> simulateDeviceDisconnection(XRDevice);
+
+  // // Simulates a user activation (aka user gesture) for the current scope.
+  // // The activation is only guaranteed to be valid in the provided function and only applies to WebXR
+  // // Device API methods.
+  // void simulateUserActivation(Function);
+};