A pack of nits in txn_clone.py and txn_doublespend.py

Summary: As per title.

Test Plan:

./test/functional/test_runner.py txn_clone txn_doublespend

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1168