[2.3] Fix phpunit test suite on travis

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10821
| License       | MIT

This is a second approach to solve #10821.
Something is weird: `Symfony\Component\Form\Tests\Extension\Core\EventListener\TrimListenerTest` passes when using PHPUnit as phar and fails when using a composer global install