Fix slow_watch_mock in test_watch_timeout_on_establishment

slow_watch_mock returns nothing, but Watcher._run expects a Watch
object returns an iterable. This causes "TypeError: 'NoneType' object
is not iterable" mentioned in
kragniz#1959 (comment)
This commit modifies slow_watch_mock so that it returns an empty list
to avoid the type error.

Author: jkawamoto

tests/test_etcd3.py

@@ -378,6 +378,7 @@ def test_watch_timeout_on_establishment(self, etcd):
 
         def slow_watch_mock(*args, **kwargs):
             time.sleep(4)
+            return []
 
         foo_etcd.watcher._watch_stub.Watch = slow_watch_mock  # noqa