Adds failing test for flushBefore in SSR

[goatslacker]

It seems like flushBefore calls ThemedStyleSheet.flush(); during render which is also executed on the server. If a component that uses SSR has the flushBefore option set then their component will fail to server render because Aphrodite can't automatically buffer without a document.

I recommend we add an environment check before we flush just to make sure the document is available.