Added txtools utility functions

Summary:
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 <contact@jasonbcox.com>

Test Plan: test_runner.py

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1891