Craft ships with PHPUnit... How can we use it to test our plugins? Since Craft ships with a copy of PHPUnit, is it possible to use that to run tests on a plugin? Or do we need to have a separate copy installed?
Yes it is possible, see this example plugin by Selvin Ortiz: https://github.com/selvinortiz/craft.loath.

You can also check out https://github.com/boboldehampsink/import for some detailed unit tests.