4.5 WebTestCase $environment BC break

### Preconditions
1. PHP 8.0
2. Symfony 5.4
2. liip/functional-test-bundle 4.5

### Steps to reproduce
1. rename Symfony test environment to testing
2. create a test like following
```
namespace App\Tests;

use Liip\FunctionalTestBundle\Test\WebTestCase;

class FunctionalTestCase extends WebTestCase
{
protected $environment = 'testing';

public function testFoo()
{
$this->assertTrue(true);
}
}

```
### Expected result
The test should pass or at least should be failing with a proper error message or deprecation warning

### Actual result
test fail with `You have requested a non-existent parameter "secret".` error message.

Start by reproducing the failure with PHP 8.0, Symfony 5.4, liip/functional-test-bundle 4.5, and a renamed testing environment, using the WebTestCase example in the issue. Trace how WebTestCase handles the custom $environment and the request for the secret parameter. Done means the test passes or reports a proper error or deprecation warning instead of the current non-existent parameter error.

Written by the indexing model from the issue text.