pytest wrapper should return None, not result

While the wrapper does need the output of the wrapped function for
processing, pytest complains for any test function that returns data.

Updates the wrapper function that processes mocked data to not return
the wrapped function result.

Closes #2118

napalm/base/test/getters.py

@@ -90,7 +90,7 @@ def mock_wrapper(cls, test_case):
             attr.current_test = ""  # Empty them to avoid side effects
             attr.current_test_case = ""  # Empty them to avoid side effects
 
-        return result
+        return None
 
     @functools.wraps(func)
     def real_wrapper(cls, test_case):