docstr: Test that the progress handler is invoked once it is set.
code: def test_progress_handler_used(self):
      """
      Test that the progress handler is invoked once it is set.
      """
      progress_calls = []
  
      def progress():
          progress_calls.append(None)
          return 0
  
      self.con.set_progress_handler(progress, 1)
      self.con.execute(
          """
              create table foo(a, b)
              """
      )
      self.assertTrue(progress_calls)
  
imports: import contextlib
  import sqlite3
  import unittest
  from test.support.os_helper import TESTFN, unlink
  from .util import memory_database, cx_limit, with_tracebacks
  from .util import MemoryDatabaseMixin
label_desc: Use image node_id 1 for calling the ProgressTests obj's underlying member method code with example usage: obj.test_progress_handler_used() and returns: int