Rename rpc-tests directory to functional
Summary:
Port over the following commit from Bitcoin core:
commit c28ee91db07ce82e134d500ddeb5600363c98048 Author: John Newbery <john@johnnewbery.com> Date: Thu Mar 9 09:44:57 2017 -0500 Rename rpc-tests
Depends on D829
Relates to T124
Test Plan: test/rpc-tests/test_runner.py
Reviewers: deadalnix, Yura, jasonbcox, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D831