Unish, Drush's test suite, makes use of the composer autoloader.

Base *TestCase classes have moved and been renamed, as they are now in a 'Unish' namespace. 

Any custom test classes need to change to new namespace and class names. If you have customized tests/phpunit.xml, you should edit its bootstrap entry to match tests/phpunit.xml.dist.

I'll wait a couple days before merging this. Feedback welcome.