Use None as the getattr default, not True

The default here should be None, not True, otherwise
this test does not actually test for the `_testing`
attribute value.

tests/test_callbacks.py

@@ -4,4 +4,4 @@ def test_when_imagej_starts(ij):
     when_imagej_starts, which injects a small piece of data into the gateway
     object. We check for that data here to make sure the callback happened.
     """
-    assert True is getattr(ij, "_testing", True)
+    assert True is getattr(ij, "_testing", None)