code: def test_add_root(self):
      fake_opr = MagicMock()
      fake_opr.children = []
  
      opt_ctxt = OptimizerContext(MagicMock(), CostModel())
      opt_ctxt.add_opr_to_group(fake_opr)
      self.assertEqual(len(opt_ctxt.memo.group_exprs), 1)
  
imports: import unittest
  from mock import MagicMock
  from evadb.optimizer.cost_model import CostModel
  from evadb.optimizer.optimizer_context import OptimizerContext
label_desc: Use image node_id 1 for calling the TestOptimizerContext obj's underlying member method code with example usage: obj.test_add_root() without return types