[LINTER] Move phpcs ruleset to a test/lint/phpcs directory

Summary:
Having the ruleset file located near the arcanist sources is not ideal.
Moving it to its own directory under test/lint/phpcs makes more sense.

Depends on D3287

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3288