Modify the test_node.py `start()` method. Before the
`subprocess.Popen()` call, add the following line:
`assert("-greatwallactivationtime=1546300800" in self.args)`
Run `./test/functional/test_runner.py <test>` on any test and check it
fails due to the newly added assertion.
Run `./test/functional/test_runner.py --extended --with-greatwallactivation` and check there is no error.