input: Write a python function to add data to a Tree object and verify its children and data.
  assert len(tree.children) == 1
  assert tree.get_data() == "Root"
  assert tree.children[0].get_data() == "Child"
output: class Tree:
      def __init__(self, data):
          self.data = data
          self.children = []
  
      def add_child(self, child_tree):
          self.children.append(child_tree)
  
      def get_data(self):
          return self.data
  
  def test_tree():
      tree = Tree("Root")
      child = Tree("Child")
      tree.add_child(child)
  
      assert len(tree.children) == 1
      assert tree.get_data() == "Root"
      assert tree.children[0].get_data() == "Child"