test: avoid unneeded txid hex -> integer conversions

Summary:
Rather than determining a CTransaction's txid as an integer by
converting it's hex value, it can be directly accessed via the
introduced .txid_int property.

This concludes backport of core#32421
https://github.com/bitcoin/bitcoin/pull/32421/commits/4ef6253017672a74c584e6e9b449ffff79f67273

Depends on D18715

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18716