While testing the fix for JDK-8260528, I discovered that we didn't have a test that displayed an initially iconified Stage and verified that it was, in fact, iconified. A recently added test has now accidentally done this, but we should have a more explicit test for all of these attributes. At least the following are needed: iconified, maximized, fullscreen.

The tests can probably go in javafx.graphics in the test.javafx.stage package (unless some robot interaction is needed).