test: Move linters to test/lint, add readme

Summary:
Backport of core PR13281
https://github.com/bitcoin/bitcoin/pull/13281/files

Not ported:

• Travis changes, irrelevant
• Functional tests doc update, as we don't use a script for the python

linter but arcanist.

Test Plan: Read the doc.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3703