Add two simple scenarios to test childre iframes

First scenario tests the injection of the client on child iframe. The
child iframe is made annotatable, by having an
`enable-annotation="true"` attribute and sharing the same origin. The
injection of the annotator happens after the the sidebar is created,
thus testing if the child iframe is able to discover the sidebar iframe
and communicate with it via postMessages.

In the second scenario, the host page contains two children iframes,
each with its own client. Every annotator/sidebar combo on each frame
should work independently from each other and shouldn't pass any message
across frames.