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.
@aik099 the current driver implementation would return ``/html//footer//form[1]``. But they could return ``/html//footer//form`` and match the same element for most features (the only broken feature would be searching for elements inside it). The goal of this new test is precisely to ensure that drivers are not implemented in a broken way