[TwigBundle] Fixed test on PreviewErrorController

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

Fixed this error in tests: https://travis-ci.org/symfony/symfony/jobs/43158442#L1534