new_contents: import unittest
  
  from mock import Mock, patch
  from patroni.async_executor import AsyncExecutor
  from threading import Thread
  
  
  class TestAsyncExecutor(unittest.TestCase):
  
      def setUp(self):
          self.a = AsyncExecutor()
  
      @patch.object(Thread, 'start', Mock())
      def test_run_async(self):
          self.a.run_async(Mock(return_value=True))
  
      def test_run(self):
          self.a.run(Mock(side_effect=Exception()))
  
subject: add missing tests for async_executor