[tests] Functional test naming convention

Summary:

• [tests] [docs] update README for new test naming scheme
• [tests] README.md nit fixes
• [tests] Check tests conform to naming convention

Extra-Author: John Newbery <john@johnnewbery.com>

This is a backport of Core PR11796

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5750