code: def test_add_columns(self):
      t = self.t_env.from_elements(
          [(1, "Hi", "Hello")], ["a", "b", "c"]
      )
      result = t.select(t.a).add_columns(
          (t.a + 1).alias("b"), (t.a + 2).alias("c")
      )
      query_operation = result._j_table.getQueryOperation()
      self.assertEqual(
          "[a, plus(a, 1), " "plus(a, 2)]",
          query_operation.getProjectList().toString(),
      )
  
imports: from pyflink.testing.test_case_utils import PyFlinkStreamTableTestCase
label_desc: Use image node_id 1 for calling the StreamTableColumnsOperationTests obj's underlying member method code with example usage: obj.test_add_columns() without return types