Add input[directory] test

Here's a v3 test, based on the recommendations in #675 (which hasn't seen activity in a while).

All attribute names except `webkitdirectory` and `directory` are guesses for now, as only Webkit implements this attribute (thought, Mozilla should [have an implementation soon](https://bugzilla.mozilla.org/show_bug.cgi?id=846931)).

Let me know if there's anything wrong with it.