Add regression test for nested definitions

The bug was already fixed since it also affected E302 but this adds a
test to ensure it doesn't regress for nested definitions either.

Closes gh-619

Author: sigmavirus24

testsuite/E30not.py

@@ -158,3 +158,7 @@ class Bar(object):
 #: Okay
 defaults = {}
 defaults.update({})
+#: Okay
+def foo(x):
+    classification = x
+    definitely = not classification