[CI] Run functional tests both pre and post graviton

Summary:
This diff moves the -greatwallactivation option of the test framework
to a similar -gravitonactivation and updates the CI.

This is the graviton version of D2518 and D2460.

Test Plan:
Modify the test_node.py start() method. Before the
subprocess.Popen() call, add the following line:
assert("-gravitonactivationtime=11546300800" in self.default_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-gravitonactivation` and check there is no error.

Read the Teamcity build log and check the functional tests are run
twice.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3068