Added an extra test to ensure the right behavior for traversal

Technically, using the provided XPath in a driver for the first matched element rather than using a restriction of this element to the first match will give you the same element match for ``find()`` (as it will get the first match only). But it can break searching for children elements (it would search them in all matches for ``footer form`` rather than only the first match).
By curiosity, I broke BrowserKitDriver to do exactly this and ran the testsuite. And it was passing, which is bad. So here is the missing test to ensure that ``$element->findAll()`` searches only in the right element, and not in all previous matches.
That's why I said that I like that approach.