Start the server.

def _start(self):
        """Start the server."""
        assert self._tempdir is not None
        assert self._process is None
        self._process = subprocess.Popen(
            [
                self.paths.slapd,
                '-f', self._slapd_conf,
                '-h', self.uri,
                '-d', str(self.slapd_debug),
            ],
            stdout=sys.stdout,
            stderr=sys.stderr,
        )
        self._poll_slapd(timeout=self.max_server_startup_delay)