Rename hash to txid for COutPoint in the test framework

Summary:
This fixes two issues:

1. hash as used in COutPoint init() shadows a python builtin.
2. hash is actually txid in the rest of the code base.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13670