LPS-44452 test skeleton

Hey @Preston-Crary 

the solution looks perfect to me. I just moved the code to a separated method in order to make more explicit what is going on, but I didn't change the logic.

I reviewed the change with @epgarcia, the leader of page/site templates and he mentioned that we should write a test to cover this use case. There are a ton already covering similar situations, so I just added another commit (that you can undo) starting the test so that you can see how it works and write one yourself. It should fail without your changes and pass with them. In order to run it, you just have to do: ant test-class -Dtest.class=LayoutSetPrototypePropagationTest