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.

tox.ini

@@ -23,6 +23,7 @@ deps =
   pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
   pytestfeatures: git+https://github.com/pytest-dev/pytest.git@features
   pexpect: pexpect
+  filelock
 platform=
   pexpect: linux|darwin
 commands=