[Tests] Rename create_tx and move to blocktools.py
Summary:
Partial backport of Core PR13669
https://github.com/bitcoin/bitcoin/pull/13669/commits/157651855f91c3c093c27290a349a231ac5ba740
Test Plan:
test_runner.py mempool_reorg mempool_resurrect mempool_spend_coinbase
Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D4820