code: def test_markdown_cell(self):
      tc = new_text_cell("markdown", "hi")
      self.assertEqual(tc.source, "hi")
  
imports: from __future__ import annotations
  from unittest import TestCase
  import pytest
  from nbformat.v3.nbbase import NotebookNode, nbformat, new_author, new_code_cell, new_heading_cell, new_metadata, new_notebook, new_output, new_text_cell, new_worksheet
label_desc: Use image node_id 7 for calling the TestCell obj's underlying member method code with example usage: obj.test_markdown_cell() without return types