Some things to consider when testing a SharePoint solution:
- At the end of the day, a SharePoint solution is just a website, so test it as such (supported browsers, OSs, etc.)
- Test as per the end-user requirements. If the solution is an external website, focus testing as an average user accessing the page. If the solution is an intranet with a large amount of document collaboration, focus testing on that functionality.
- Focus testing on the custom parts of the SharePoint solution. If a Web Part is out-of-the-box just reconfirm that it is configured properly and is working, but do not focus on it. If the Web Part is a custom component built for the solution from the ground up, focus the testing on that Web Part.