MAINT: Add _exception_on_warning to MockApp (#586)

MockApp in test suite is now breaking due to the absence
of a private _exception_on_warning attribute.

Fix by explicitly adding to the MockApp.

numpydoc/tests/test_numpydoc.py

@@ -49,6 +49,7 @@ def __init__(self):
         self.verbosity = 2
         self._warncount = 0
         self.warningiserror = False
+        self._exception_on_warning = False
 
 
 def test_mangle_docstrings_basic():