[2.3][CssSelector] Added test to highlight a regression

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no (that's the point)
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Hello.

I tried to use latest Symfony2 version in fapot/sismo, and sismo tests fail because of a regression in CssSelector Component.

You can easily reproduce it by cloning sismo, run a `composer update`, then `phpunit`.

I tried to debug it, but DomDocument and XPath don't like me.

ping @jfsimon