Rename qa directory to test

Summary:
Port the following commit from Bitcoin Core:

commit 00902c48cdc5d6a531abfd279fd8e7779cf0a0b8
Author: John Newbery <john@johnnewbery.com>
Date:   Wed Mar 8 15:56:30 2017 -0500

    Rename qa directory to test

Depends on D828.
Relates to T124.

Test Plan: ./test/pull-tester/run-tests.py

Reviewers: deadalnix, Yura, jasonbcox, akb, #bitcoin_abc

Reviewed By: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D829