test(rxActionMenu): Tests and page objects.

Closes #525
I just realized we were talking about the `customActionMenuItem` from the test, and not the [default behavior](https://github.com/rackerlabs/encore-ui/blob/e2e-rx-action-menu/src/rxActionMenu/rxActionMenu.page.js#L4-L29), which uses the same approach. Yeah, I can change this one. But please take a look at the default behavior too, and let me know if that should use modals, too.