Adds test showing an issue when changing nowShowing during a transition

This PR adds a test to show an issue that I'm having in my app when changing nowShowing twice while the container is still transitioning.

If you run the test with the console open, you will see this error: `Uncaught Error: SC.ContainerView:sc370.removeChild(View 1) must belong to parent ` which is the same error that I have in my app.