Add composer.json

List phpunit/phpunit as development dependency.
Switch to current WP unit test suite

Courtesy of WP CLI scaffold
Define global plugin object using $GLOBALS

Fixes fatals during unit test runs.
Fix PHP warnings in test-library.php

Removes global / cache flushing functionality that is no longer needed after switch to current unit test suite.
Restructure test directories

Add note about selenium tests / private unit test suite fork.