Added txtools utility functions
Part 1 of many for incorporating changes from https://github.com/bitcoinxt/bitcoinxt/pull/487
The goal is to improve the test suite so that it doesn't fail post-fork.
This diff differs from the original primarily in variable/function naming and other minor cleanups.
Co-authored-by: Jason B. Cox <firstname.lastname@example.org>
Test Plan: test_runner.py
Reviewers: deadalnix, schancel, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D1891