private function checkIfWorkersCrashed(): void
    {
        if ($this->db->query('SELECT COUNT(id) FROM tests')->fetchColumn(0) === '0') {
            return;
        }

        throw new RuntimeException(
            'Some workers have crashed.' . PHP_EOL
            . '----------------------' . PHP_EOL
            . 'All workers have quit, but some tests are still to be executed.' . PHP_EOL
            . 'This may be the case if some tests were killed forcefully (for example, using exit()).' . PHP_EOL
            . '----------------------' . PHP_EOL
            . 'Failed test command(s):' . PHP_EOL
            . '----------------------' . PHP_EOL
            . implode(PHP_EOL, $this->db->query('SELECT command FROM tests')->fetchAll(PDO::FETCH_COLUMN))
        );
    }