Fix abc-invalid-message.py flakiness

Summary:
The test will fail occasionally, especially on OSX. This is due to a peer disconnection happening before the connection check completed.
This diff delays sending the invalid message and prevents this wrong operation ordering.

Note that I couldn't reproduce the issue locally, but at least this doesn't make things worst.

Test Plan:
In a loop:

./test/functional/test_runner.py abc-invalid-message

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D10302