[Form] Fixed ValidatorExtension to work with the 2.5 Validation API

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11606
| License       | MIT
| Doc PR        | -

The ValidatorExtension::loadTypeGuesser() method used to fail because the method `getMetadataFactory()` does not exist anymore on the new `ValidatorInterface`. This was fixed easily, because `ValidatorInterface` now extends `ClassMetadataFactory`.
using a real instance rather than a mock means that you don't test anymore that the metadata are added