Removed `test_cron_execution_upon_system_clock_change`.

The test is too brittle as it depends on small mocked clock skews and it
can easily fail.

tests/test_cron.py

@@ -1,23 +1,11 @@
-import datetime
 import queue
 import pytest
 import time
 
-from dateutil.tz import gettz
-from mock import patch
-
 test_timeout = 10
 cron_queue = queue.Queue()
 
 
-class MockDatetime(datetime.datetime):
-    timedelta = datetime.timedelta()
-
-    @classmethod
-    def now(cls):
-        return super().now(tz=gettz()) + cls.timedelta
-
-
 def _test_cron_queue(expected_msg: str):
     msg = None
     test_start = time.time()
@@ -37,27 +25,6 @@ def test_cron_execution():
     _test_cron_queue('cron_test')
 
 
-def test_cron_execution_upon_system_clock_change():
-    """
-    Test that the cronjob runs at the right time even upon DST or other
-    system clock changes.
-    """
-    # Mock datetime.datetime with a class that has overridable timedelta
-    patcher = patch('datetime.datetime', MockDatetime)
-
-    try:
-        patcher.start()
-        time.sleep(1)
-        # Simulate a +1hr shift on the system clock
-        MockDatetime.timedelta = datetime.timedelta(hours=1)
-        time.sleep(1)
-    finally:
-        patcher.stop()
-
-    # Ensure that the cronjob that was supposed to run in an hour is now running
-    _test_cron_queue('cron_1hr_test')
-
-
 if __name__ == '__main__':
     pytest.main()