Test for childViews added at an existing index

should cover
https://coveralls.io/files/369411688#L296
and
https://coveralls.io/files/369411688#L419

@ahumphreys87 I believe this covers https://github.com/marionettejs/backbone.marionette/commit/9605abe52dec1cb1f99917952e3f7c941334e854