Call server_close() on shutdown

Server threads take some times to shutdown, calling
ThreadMixin.server_close() will waits on those threads.

Author: bdauvergne

pytest_sftpserver/plugin.py

@@ -15,3 +15,4 @@ def sftpserver(request):
 
     if server.is_alive():
         server.shutdown()
+    server.server_close()