Unish, Drush's test suite, makes use of the composer autoloader to find its base TestCase classes. Any custom test classes need to change to new the class names. If you have your own tests/phpunit.xml you should edit its bootstrap entry to match tests/phpunit.xml.dist.

Conflicts:
	tests/backendTest.php
Use latest Composer as autoload-dev depends on that.
Add root namespace to a class call.
Allow for global composer install as well as local, in Unish.