[qa] fix flakiness of bip68-sequence

Summary:
Currently bip68-squence sometimes fails due to an inability to sync blocks.
This is because the block generated in activateCSV is sometimes synced to node1
from node0, and prior to it being invalidated on node0. If this happens, node0
can never sync the tip from node1 (as it has been explicitly invalidated).

The fix is to disconnect the two nodes prior to block generation on node0,
activateCSV, invalidate the block, then sync.

Test Plan:

./test/functional/test_runner.py bip68-sequence

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1805