Add testcase for cdef class method with closure

tests/run/closure_self.pyx

+cdef class SelfInClosure(object):
+    """
+    >>> o = SelfInClosure()
+    >>> o.closure_method()() == o
+    True
+    """
+
+    def closure_method(self):
+        def nested():
+            return self
+        return nested