minor #15767 [Validator] mark test as legacy (TomasVotruba)

This PR was merged into the 2.8 branch.

Discussion
----------

[Validator] mark test as legacy

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Ref https://github.com/symfony/symfony/pull/15708#discussion_r39246000

Failing test are hhvm binaries: https://travis-ci.org/symfony/symfony/jobs/79951993#L530

Commits
-------

cf18a99 [Validator] mark test as legacy

src/Symfony/Component/Form/Tests/Extension/Validator/EventListener/ValidationListenerTest.php

@@ -189,6 +189,9 @@ class ValidationListenerTest extends \PHPUnit_Framework_TestCase
         $this->assertAttributeSame($validator, 'validator', $listener);
     }
 
+    /**
+     * @group legacy
+     */
     public function testValidatorInterfaceUntilSymfony24()
     {
         // Mock of ValidatorInterface until apiVersion 2.4
@@ -199,6 +202,7 @@ class ValidationListenerTest extends \PHPUnit_Framework_TestCase
     }
 
     /**
+     * @group legacy
      * @expectedException \InvalidArgumentException
      */
     public function testInvalidValidatorInterface()