Intermittent error: Failed to shutdown the live test server in 2 seconds

Browser tests using "kalite/manage.py test main --liveserver=localhost:8004-8010" fail intermittently with "Failed to shutdown the live test server in 2 seconds"
django example code + forums suggest calling the superclass teardown method to prevent this error.

Full error is 
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ERROR: tearDownClass (main.tests.browser_tests.DeviceUnregisteredTest)
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Traceback (most recent call last):
  File "/home/pi/ka-lite/kalite/../python-packages/django/test/testcases.py", line 1145, in tearDownClass
    cls.server_thread.join()
  File "/home/pi/ka-lite/kalite/../python-packages/django/test/testcases.py", line 1069, in join
    self.httpd.shutdown()
  File "/home/pi/ka-lite/kalite/../python-packages/django/test/testcases.py", line 956, in shutdown
    "Failed to shutdown the live test server in 2 seconds. The "
RuntimeError: Failed to shutdown the live test server in 2 seconds. The server might be stuck or generating a slow response.