This will set "-greatwallactivationtime" in the past (01/01/2019).
Together with D2460, it will allow for running the functional tests both
pre and post update.
I considered using initialize_datadir() to set the argument into the
config file (like proposed in D2431) but:
- it may not work with tests that override the setup_chain() method
(currently there is only rpc_users.py, which would work anyway because
it calls super.setup_chain())
- it may not work with tests that override the datadir or config file
location. This is the case of feature_conf_args.py.
If a test sets the -greatwallactivationtime argument in the node
extra_args, or by passing them to the start_node(), it will still
override this argument due to the precedence rules.