Create a test for a LegacyApplication running using LegacyVaadinPortlet

[vaadin-bot]

Originally by @Artur-

---

Once #9566 has been fixed there should be an additional portlet test which uses LegacyVaadinPortlet.

---

Imported from https://dev.vaadin.com/ issue #9585

[stale]

A lot of tickets have been left hanging in the issue tracker through the years. Some of them are still relevant, some of them have been fixed a long time ago and some are no longer valid. To get a better look on what is important and still relevant, we are closing old tickets which have not been touched in a long time. No further work will be done on this ticket. If the ticket seems to be still actual, please verify the problem existence over latest framework version and then open a new ticket in vaadin/framework with all the suitable information.

[stale]

The issue was automatically closed due to inactivity. If you found some new details to it or started working on it, comment on the issue so that maintainers can re-open it.