Fix leak releasing child view controller. (pointfreeco#511)

When running with host application, rootViewController is retained by the window, and it retains the view controller we are testing (which is a child view controller). This causes a leak on the view controller we're testing.
By removing the view controller from it's parent, no one retains the view controller, fixing the leak.

Author: llinardos

Sources/SnapshotTesting/Common/View.swift

@@ -941,6 +941,10 @@ private func add(traits: UITraitCollection, viewController: UIViewController, to
 
   return {
     rootViewController.beginAppearanceTransition(false, animated: false)
+    viewController.willMove(toParent: nil)
+    viewController.view.removeFromSuperview()
+    viewController.removeFromParent()
+    viewController.didMove(toParent: nil)
     rootViewController.endAppearanceTransition()
     window.rootViewController = nil
   }