pressDownOnCollapsedMobileHandle method 
    
    
    
  Implementation
  Future<TestGesture> pressDownOnCollapsedMobileHandle() async {
  final handleElement = find.byKey(DocumentKeys.androidCaretHandle).evaluate().firstOrNull;
  assert(handleElement != null, "Tried to press down on Android collapsed handle but no handle was found.");
  final renderHandle = handleElement!.renderObject as RenderBox;
  final handleCenter = renderHandle.localToGlobal(renderHandle.size.center(Offset.zero));
  final gesture = await startGesture(handleCenter);
  return gesture;
}