test(cdk/testing): use offsetX and offsetY to verify element coordinates

Currently a couple of the tests use `clientX` and `clientY` plus the element's `ClientRect` to verify click coordinates relative to an element. This can cause flakes in tests, because the `ClientRect` changes depending on the page's scroll position.

These changes use `offsetX` and `offsetY` instead which are actually relative to the element.

Author: crisbeto

src/cdk/testing/tests/test-main-component.ts

@@ -123,8 +123,7 @@ export class TestMainComponent implements OnDestroy {
   }
 
   private _assignRelativeCoordinates(event: MouseEvent, obj: {x: number; y: number}) {
-    const {top, left} = this.clickTestElement.nativeElement.getBoundingClientRect();
-    obj.x = Math.round(event.clientX - left);
-    obj.y = Math.round(event.clientY - top);
+    obj.x = Math.round(event.offsetX);
+    obj.y = Math.round(event.offsetY);
   }
 }