[tests] Remove unused optional verify_checksum parameter

Summary:
Commit description:

This optional parameter is never used, so remove it.

PR description:

Lots of small fixes:

moving unit tests to test_framework implementation files
renaming functions to be clearer
removing multiple imports
removing unreadable byte literals from the code
fixing pep8 violations
correcting out-of-date docstring

Most of the PR is related to segwit or code style, so only 2 out of 7 commits are relevant.

This is a backport of core#19253 [1/2]
https://github.com/bitcoin/bitcoin/pull/19253/commits/7f639df0b8a15aaeccedab00b634925f568c2c9a

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10417