Support Symfony 4.3

• Removed method getClient from Kununu\TestingBundle\Test\WebTestCase.

Before:

$response = $this->doRequest(
    $this->getClient(),
    RequestBuilder::aGetRequest()->withUri('/app/response')
);

After:

$response = $this->doRequest(
    RequestBuilder::aGetRequest()->withUri('/app/response')
);