Piwik unit tests suite can be found in the Piwik SVN:
http://dev.piwik.org/svn/trunk/tests/

You can run all tests 
http://dev.piwik.org/svn/trunk/tests/all_tests.php

You can run one test file at a time by going to the test file directly 
http://dev.piwik.org/svn/trunk/tests/core/DataTable.test.php

Piwik tests use the Simpletest Unit Testing framework.

If you want to help, we are always looking to improve the Piwik code coverage!  
Please contact us at hello@piwik.org
 
