Can't overwrite WebTestCase $env static property

### Preconditions

1. PHP 8.0
2. Symfony 5.4
3. 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 static $env = 'testing';

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

### Expected result
The test should pass

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

### Proposed solution
Replace `self::$env` with `static::$env` in related files (see: https://github.com/liip/LiipFunctionalTestBundle/pull/593/files)

Start with Liip\FunctionalTestBundle\Test\WebTestCase and review the related files shown in pull request #593. Reproduce the case with PHP 8.0, Symfony 5.4, and the renamed testing environment; the work is done when the functional test passes without the non-existent "secret" parameter error.

Written by the indexing model from the issue text.