docstr: Tests that adding a node to the original graph does not
  affect the nodes of the subgraph.
code: def test_add_node(self):
      """Tests that adding a node to the original graph does not
      affect the nodes of the subgraph.
  
      """
      self.G.add_node(5)
      assert [0, 1, 3, 4] == sorted(self.H.nodes)
      self.G.remove_node(5)
  
imports: import pytest
  import networkx
  from networkx.utils import edges_equal
label_desc: Use image node_id 4 for calling the TestEdgeSubGraph obj's underlying member method code with example usage: obj.test_add_node() without return types