In the last few months we were tasked to create test cases including a test matrix for what was developed in our SharePoint 2010 solution. So I started out on my Google Search trying to find a generic test document which I can reuse across my site as I did not want to write any test cases for the OTB functionality that SharePoint offers.
I found this link to provide good guidance in formulating a test plan as well as a test matrix document.
There are various other sources available on the Internet, but I have found the one mentioned above most useful for my specific requirements.
I hope this helps :-)