Expand event-handler-attributes-body-window.html

The test at html/webappapis/scripting/events/event-handler-attributes-body-window.html could be expanded. For example, currently it tests

    document.body['on' + handler] = f;
    assert_equals(window['on' + handler], f);

while it could also test the opposite direction:

    window['on' + handler] = f;
    assert_equals(document.body['on' + handler], f);

Also, it would be useful to test with:

• a frameset as the body
• a frameset or body element that's not in a document
• a frameset or body element that's not the body element (for example, with multiple body elements in the DOM).