[qa] add monolithactivationtime to mempool reorg to ensure it works

Summary:
The existing test framework mines 200 blocks starting Jan 1 2014.
Then this test mines some additiona blocks in the current time,
then it invalidates one. This causes monolith activation to trip
and clear the mempool during the test, when one of the current-time
blocks is invalidated.

Test Plan:

./test/functional/test_runner.py mempool_reorg.py

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1423