Use object returned from add_p2p_connection to refer to
p2ps. Add a test class attribute if it needs to be used across
many methods. Don't use the p2p property.
Remove the BIP61 REJECT code from error messages and logs when a
transaction is rejected.
BIP61 support was removed from Bitcoin Core in
fa25f43ac5. The REJECT codes will be
removed from the codebase entirely in the following commit.