[schnorr functional test] improve tx handling

Summary:
This strengthens the p2p nature of the test: just before we are
disconnected, we get a reject reason, which we can test for.
Accordingly, the reject code handling has been upgraded so we can
match the RPC error against the p2p error.

Also, when sending valid transactions, send them over p2p connection
instead and get the node to mine them, to demonstrate they are fully
acceptable transactions.

Depends on D3615

Test Plan: ./test/functional/test_runner.py abc-schnorr.py

Reviewers: #bitcoin_abc, Mengerian, Fabien, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3617