fix: correct always-true state check in DockerVM.stop()

The condition 'state != "stopped" or state != "exited"' is a tautology,
so the state check was a no-op and a stop request was sent even for a
container that had already exited.

_get_container_state() never returns "stopped" (only "running",
"paused" or "exited"), so the intended negation of the condition used in
_fix_permissions() requires 'and', not 'or' (De Morgan's law).

Added a regression test asserting no stop query is issued for an
already-exited container.

gns3server/compute/docker/docker_vm.py

@@ -869,7 +869,7 @@ class DockerVM(BaseNode):
                 await self._fix_permissions()
 
             state = await self._get_container_state()
-            if state != "stopped" or state != "exited":
+            if state != "stopped" and state != "exited":
                 # t=5 number of seconds to wait before killing the container
                 try:
                     await self.manager.query("POST", "containers/{}/stop".format(self._cid), params={"t": 5})