I would like to add and encourage that all new developments that come
with the SMW 1.9 branch ought to come with its own test
+1. Tests make life easier for developers and increase quality of the software.
These are some notes on existing components that can use unit tests which I mainly wrote for Nischay earlier this year. An actual overview of what PHPUnit tests are and how they work in MediaWiki can be found here: https://www.mediawiki.org/wiki/Manual:PHP_unit_testing
Please note that although our DataValues and DataItems might lack tests at the moment, it might make more sense to wait on the planned refactor, which will change them to make use of the DataValues extension, which is fully unit tested. That's not for SMW 1.9 yet though. More on that here http://meta.wikimedia.org/wiki/Wikidata/Notes/SMW_and_Wikidata
So I'd focus on other parts of the codebase now if you want to add tests for existing code.
Jeroen De Dauw
Don't panic. Don't be evil.