[DomCrawler] fix the axes handling in a bc way

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11503
| License       | MIT
| Doc PR        | 

The previous fix in #11548 for handling XPath axes was not backward compatible. In previous Symfony versions the Crawler handled nodes by holding a "fake root node". This must be taken into account when evaluating (relativizing) XPath expressions.
Yes, ``child::foo`` is equivalent to ``./foo`` (which is covered with a similar test in ``testFilterXpathComplexQueries()``).

I applied the tests on Symfony 2.3.13 in [this branch](https://github.com/xabbuh/symfony/tree/pr-11624-2.3.13-tests). The only tests being skipped are the ones affecting the fake root node and the namespace axes. The results are [here](https://travis-ci.org/xabbuh/symfony/builds/32367887).