Fix User.stop() crash when greenlet hasn't started yet

[reachsridhard]

Fix User.stop() for users whose greenlet has not started

Summary

Fixes User.stop(force=False) raising an exception when a user's greenlet has been spawned but has not started running yet (_state is None).

This can occur during spawning when a test is stopped with a non-zero --stop-timeout.

Fix

Treat _state is None the same as LOCUST_STATE_WAITING and kill the greenlet immediately:

- if force or self._state == LOCUST_STATE_WAITING:
+ if force or self._state in (None, LOCUST_STATE_WAITING):
      self._group.killone(self._greenlet)
      return True

A user in this state has not started running, so there is no task or on_start hook to wait for.

Testing

Added test_stop_user_before_greenlet_starts to reproduce the issue.