Sling already has fairly good test coverage, but there's always room for improvement.
The goal of this Google Summer of Code project is to create more unit and integration tests for Sling.
Interested students are welcome to get in touch with the Sling community via its mailing lists to discuss the project, see http://sling.apache.org/project-information.html#mailing-lists
The student will have to get familiar with the Sling codebase, identify areas where tests are missing, write unit and integration tests to improve the test coverage, and submit the result as patches that the Sling committers can verify and hopefully apply.
Another aspect is measuring and reporting (automatically, ideally as part of our Hudson builds) the actual test coverage - that's easy for unit tests which run as part of the module builds, but seems to be harder for integration tests which run in a separate JVM than the one that executes tests.