Fixes Validator Testcase

[Hanmac]

Subject

I am targeting this branch, because i don't know if BC or not.

It fixes this:

2x: Passing a null message when instantiating a "Symfony\Component\Validator\ConstraintViolation" is deprecated since Symfony 4.4.
1x in UniqueUrlValidatorTest::testValidateWithPageFound from Sonata\PageBundle\Tests\Validator
1x in UniqueUrlValidatorTest::testValidateWithRootUrlAndNoParent from Sonata\PageBundle\Tests\Validator

The main reason for this was the faulty Translator Mock used.
I updated the tests to check for the Violations.

Closes #{put_issue_number_here}.

Changelog

### Changed
- Updated ValidatorTest to use ConstraintValidatorTestCase

[VincentLanglet]

I am targeting this branch, because i don't know if BC or not.

Since all the deprecations will only be solved on 4.x. You can do most of all the PR on 4.x, it's easier.
The only needed PR on 3.x are the one which deprecate some PageBundle code.

[jordisala1991]

Except for the translations all good!

[VincentLanglet]

Thanks