Can't overwrite WebTestCase $env static property

[mikysan]

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)