Added failing descendant-or-self XPath specs.

These currently fail due to the child:: selector not working entirely as it
should be. Consider the following XML:

    <a><b><b><c class="x"></c></b></b></a>

And the following XPath:

    descendant-or-self::node()/a

In Nokogiri/libxml this will return a node set containing the <a> node. In Oga
however this will return an empty node set. This will require some further
investigation to see what exactly is going on, and in particular what is the
correct behaviour.
Added initial CSS evaluation tests.