WebTestCase and FixturesTrait define the same property ($containers)

Nobody has claimed this yet.

I am trying to upgrade my app to Symfony 5.3. Previously this worked in 5.2.11 with functional-test-bundle 4.4.0 and test-fixtures-bundle 1.11.2. After upgrading to most recent versions of all, I get the error below:

`PHP Fatal error: Liip\FunctionalTestBundle\Test\WebTestCase and Liip\TestFixturesBundle\Test\FixturesTrait define the same property ($containers) in the composition of App\Tests\AbstractFixtureWebTestCase. However, the definition differs and is considered incompatible. Class was composed in /var/www/symfony/tests/AbstractFixtureWebTestCase.php on line 14`

This is the same error as #548 but obviously - much more recent versions.

### Preconditions

1. PHP 7.4.22
2. Symfony 5.3.4
2. liip/functional-test-bundle 4.4.2
3. liip/test-fixtures-bundle 1.12.0

### Steps to reproduce
```php
namespace App\Tests;

use Liip\FunctionalTestBundle\Test\WebTestCase;
use Liip\TestFixturesBundle\Test\FixturesTrait;

abstract class AbstractFixtureWebTestCase extends WebTestCase
{
use FixturesTrait;
```

### Expected result

1. these two bundles should work together as before

### Actual result

`PHP Fatal error: Liip\FunctionalTestBundle\Test\WebTestCase and Liip\TestFixturesBundle\Test\FixturesTrait define the same property ($containers) in the composition of App\Tests\AbstractFixtureWebTestCase. However, the definition differs and is considered incompatible. Class was composed in /var/www/symfony/tests/AbstractFixtureWebTestCase.php on line 14`

Start with the composition in tests/AbstractFixtureWebTestCase.php and inspect the $containers definitions in Liip\FunctionalTestBundle\Test\WebTestCase and Liip\TestFixturesBundle\Test\FixturesTrait. Reproduce the PHP fatal error with the listed Symfony and bundle versions, then verify that the two test helpers compose without an incompatible property definition.

Written by the indexing model from the issue text.