Use a single node in high_priority_transaction test to prevent sync problems

Summary:
The test failed irregularly. See T103.

This was probably caused by transactions being synced between node 0 and node 1 after/during the block generation on node 1.

Changed to use a single node.

Test Plan:
I can't confirm it works, as it was failing very rarely on my setup.

Please test on the travis instant with >10% failure rate.

Reviewers: freetrader, O1 Bitcoin ABC, #bitcoin_abc, sickpig, deadalnix

Reviewed By: freetrader, O1 Bitcoin ABC, #bitcoin_abc, sickpig, deadalnix

Subscribers: sickpig

Differential Revision: https://reviews.bitcoinabc.org/D551