Receive `length` bytes from a socket object.

def mock_server_receive(sock, length):
    """Receive `length` bytes from a socket object."""
    msg = b''
    while length:
        chunk = sock.recv(length)
        if chunk == b'':
            raise socket.error(errno.ECONNRESET, 'closed')

        length -= len(chunk)
        msg += chunk

    return msg