class_docstr: Test ability to marshall link_button protos.
docstr: Test that file url exist in proto.
code: def test_url_exist(self):
      """Test that file url exist in proto."""
      st.link_button("the label", url="https://streamlit.io")
  
      c = self.get_delta_from_queue().new_element.link_button
      self.assertTrue("https://streamlit.io" in c.url)
  
file_path: streamlit/lib/tests/streamlit/elements/link_button_test.py
imports: import streamlit
  from tests.delta_generator_test_case import DeltaGeneratorTestCase
label_desc: Use image node_id 3 for calling the LinkButtonTest obj's underlying member method code with example usage: obj.test_url_exist() without return types