Use a single node in mining_prioritisetransaction

Summary: The second node is ran only for the getblocktemplate call (that requires a peer connection to work). Instead of spinning a node (and running sync_all on each block) we can just add a test framework peer before the call to achieve the same result.

Test Plan:

./test/functional/test_runner.py mining_prioritisetransaction

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16412