Test "update a style block" early return connected vs. BC-connected

[domenic]

Context: whatwg/html#6294, which inserts an early return for when the element is not connected.

http://wpt.live/shadow-dom/ShadowRoot-interface.html, in particular

ShadowRoot.styleSheets must return a StyleSheetList sequence containing the shadow root style sheets when shadow root is open.

and

ShadowRoot.styleSheets must return a StyleSheetList sequence containing the shadow root style sheets when shadow root is closed.

tests shadowRoot.styleSheets and styles[i].sheet when the element is disconnected.

It would be good to add counterpart tests for when the element is connected, but not browsing-context connected. For example, when it's in a document created by document.implementation.createDocument(). Per the updated spec, such cases should behave differently than the disconnected case; i.e. they should not return 0-length / null.

/cc @emilio @annevk