StationServer.stop() race conditions

[lalten]

Repeatedly starting and stopping StationServer (e.g. in unit tests) reveals some race conditions in OpenHTF and Tornado.

1. In openhtf/output/servers/web_gui_server.py:160: AttributeError: 'StationServer' object has no attribute 'server'
   This happens if you call stop() on the server before the run() thread has finished setting up the server

2. In tornado/tcpserver.py:260: KeyError: 11
   I'm not sure why you'd hit this, but it also happens sometimes when you call StationServer's stop() method.

One workaround is to wrap it in a new context manager like

import contextlib
import time
import typing
import openhtf.output.servers.station_server

@contextlib.contextmanager
def safe_station_server() -> typing.Generator[openhtf.output.servers.station_server.StationServer, None, None]:
    """Workaround for race conditions in StationServer and Tornado libraries."""
    server = openhtf.output.servers.station_server.StationServer()
    server.start()
    while not hasattr(server, "server"):
        time.sleep(0)
    yield server
    try:
        server.stop()
    except KeyError:
        pass