Fix iterating over Mock objects

These are used in tests, so tests that iterate over something
that has been mocked out could iterate forever.

Fixes #1441.

nengo/utils/testing.py

@@ -25,6 +25,9 @@ def __call__(self, *args, **kwargs):
     def __getitem__(self, key):
         return Mock()
 
+    def __iter__(self):
+        return iter([])
+
     def __mul__(self, other):
         return 1.0
 

nengo/utils/tests/test_testing.py

@@ -72,3 +72,10 @@ def test_logger_norecord():
         logger.info("Testing that logger doesn't record")
     with pytest.raises(ValueError):
         logger_obj.get_filepath(ext='txt')
+
+
+def test_mock_iter(plt):
+    fig = plt.figure()
+    for i, ax in enumerate(fig.axes):
+        assert False, "Mock object iterating forever"
+    plt.saveas = None