Issue node shutdown on empty workload

Remote pytest_runtestloop requires at least 2 tests, or a test
and a shutdown command. In case of inter test locking, for
example using a file lock, the tests could deadlock, since the
shutdown command would not be enqueued, and the last test of a
worker would never finish, and would not allow another worker
to make progress.

xdist/scheduler/loadscope.py

@@ -306,6 +306,7 @@ class LoadScopeScheduling(object):
 
         # Check that more work is available
         if not self.workqueue:
+            node.shutdown()
             return
 
         self.log("Number of units waiting for node:", len(self.workqueue))