[Config] Fix NodeBuilderTest::testNumericNodeCreation.

[Config] Fixes a test failure, under both php and hhvm.

```
PHP Fatal error:  Class 'Symfony\Component\Config\Tests\Definition\Builder\NodeBuilder' not found in /home/d/dev-fork/symfony-fork/src/Symfony/Component/Config/Tests/Definition/Builder/NodeBuilderTest.php on line 82
```

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A