qa: Avoid checking reject code for now

Summary:

The node will often disconnect before sending a reject code. A more
robust solution would be to read from the debug log.

Backport from core PR13352
https://github.com/bitcoin/bitcoin/pull/13352/files

Should fix the CI spurious p2p_invalid_tx issue.

Test Plan:

./test/functional/test_runner.py p2p_invalid_tx

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3746