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/each.py

@@ -126,6 +126,7 @@ class EachScheduling(object):
             if not pending:
                 pending[:] = range(len(self.node2collection[node]))
                 node.send_runtest_all()
+                node.shutdown()
             else:
                 node.send_runtest_some(pending)
             self._started.append(node)

xdist/scheduler/load.py

@@ -178,6 +178,9 @@ class LoadScheduling(object):
                     return
                 num_send = items_per_node_max - len(node_pending)
                 self._send_tests(node, num_send)
+        else:
+            node.shutdown()
+
         self.log("num items waiting for node:", len(self.pending))
 
     def remove_node(self, node):

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))