improves the stability of the testWaitForFinishingAnimation-test on iOS ...

It may occur that a transition-animation (like a push-on-navigationcontroller-animation) may already be finished (= none of the CALayers has animations), but in fact, the view-hierarchy still contains all views that were shown before the transition started and the views that were shown when the transition is finished. In this state, doing a tap by calling tapScreenAtPoint: to, i.e. trigger a gesture-recognizer on a view at a certain point, may fail as the routine in tapScreenAtPoint: can't find the right view to send the tap within this in-between-transition-animation-state-view-hierarchy.
An indicator of this state is the existence of a _UIParallaxDimmingView-instance within the view-hierarchy, so if one of them is found, i assume that there is an unfinished animation and wait until it disappeared.

Here is an example of such a view-hierarchy (it is taken from a run of testWaitForFinishingAnimation)
```
<UIWindow>
   | <UILayoutContainerView>
   |    | <UINavigationTransitionView>
   |    |    | <UIViewControllerWrapperView>
   |    |    |    | <UIView>    <---- This is the root-view of the previous viewcontroller
   |    |    |    |    | <UIScrollView>
   |    |    |    |    |    | <UISegmentedControl>
   |    |    |    |    |    |    | <UISegment>
   |    |    |    |    |    |    |    | <UISegmentLabel>
   |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    | <UISegment>
   |    |    |    |    |    |    |    | <UISegmentLabel>
   |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    | <UISlider>
   |    |    |    |    |    |    | <UIView>
   |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    | <UITextField>
   |    |    |    |    |    |    | <_UITextFieldRoundedRectBackgroundViewNeue>
   |    |    |    |    |    |    | <UITextFieldLabel>
   |    |    |    |    |    | <UIRoundedRectButton>
   |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    | <UIButtonLabel>
   |    |    |    |    |    | <UILabel>
   |    |    |    |    |    | <UISwitch>
   |    |    |    |    |    |    | <_UISwitchInternalViewNeueStyle1>
   |    |    |    |    |    |    |    | <UIView>
   |    |    |    |    |    |    |    |    | <UIView>
   |    |    |    |    |    |    |    | <UIView>
   |    |    |    |    |    |    |    |    | <UIView>
   |    |    |    |    |    |    |    | <UIView>
   |    |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    | <UIRoundedRectButton>
   |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    | <UIButtonLabel>
   |    |    |    |    |    | <UITextField>
   |    |    |    |    |    |    | <_UITextFieldRoundedRectBackgroundViewNeue>
   |    |    |    |    |    | <UIScrollView>
   |    |    |    |    |    |    | <UIRoundedRectButton>
   |    |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    |    | <UIButtonLabel>
   |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    | <UILabel>
   |    |    |    |    |    | <UILabel>
   |    |    |    |    |    | <UILabel>
   |    |    |    |    |    | <UILabel>
   |    |    |    |    |    | <UIRoundedRectButton>
   |    |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    |    | <UIButtonLabel>
   |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    | <UIImageView>
   |    |    |    | <_UIParallaxDimmingView>
   |    |    |    | <UIView>
   |    |    |    |    | <_UIParallaxDimmingView>
   |    |    |    |    |    | <UIImageView>
   |    |    |    |    |    | <UIView>      <---- This is the root-view of the pushed viewcontroller
   |    |    |    |    |    |    | <UILabel>
   |    | <UINavigationBar>
   |    |    | <_UINavigationBarBackground>
   |    |    |    | <UIImageView>
   |    |    | <UINavigationItemButtonView>
   |    |    |    | <UILabel>
   |    |    | <_UINavigationBarBackIndicatorView>
```