Fixed #22531 -- Added tree.Node.__repr__ and tests for the class.

While Node class has a useful `__str__`, its `__repr__` is not that
useful. Added a `__repr__` that makes use of the current `__str__`.
This is especially useful since the more popular `Q` class inherits
`tree.Node`. Also created new tests that cover most of `Node` class
functionality.

django/utils/tree.py

@@ -48,6 +48,9 @@ class Node(object):
         return '(%s: %s)' % (self.connector, ', '.join([str(c) for c in
                 self.children]))
 
+    def __repr__(self):
+        return "<%s: %s>" % (self.__class__.__name__, self)
+
     def __deepcopy__(self, memodict):
         """
         Utility method used by copy.deepcopy().