Merge #14816: Add CScriptNum decode python implementation in functional suite

Summary:
2012d4df2 Add CScriptNum decode python implementation in functional suite (Gregory Sanders)

Pull request description:

I needed this for reasons and thought it'd be good to upsteam it.

Tree-SHA512: 6ea89fa2a5f5a7759ba722f2b4ed5cd6423ebfff4e83ac8b8b5c935e6aa479684e626c5f41fa020816d2a9079a99af5564e30808594d5c13e3b51ec9b474926d

Backport of Core PR14816

Test Plan: test_runner.py mining_basic

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6713