chore(testing): fix compilation for TS3.6. (#17218)

TS3.6 no longer includes the initUIEvent method.

Author: mprobst

src/cdk/testing/event-objects.ts

@@ -61,7 +61,8 @@ export function createTouchEvent(type: string, pageX = 0, pageY = 0) {
   const event = document.createEvent('UIEvent');
   const touchDetails = {pageX, pageY};
 
-  event.initUIEvent(type, true, true, window, 0);
+  // TS3.6 removes the initUIEvent method and suggests porting to "new UIEvent()".
+  (event as any).initUIEvent(type, true, true, window, 0);
 
   // Most of the browsers don't have a "initTouchEvent" method that can be used to define
   // the touch details.