Testing seems fairly straightforward, however, it can be quite complex. Using technologies such as Windows installer provides you reduced testing, as you can assume any of the code you're making use of in Windows installer is already tested. You don't need to test the act of copying files, or the act of setting/changing registry values work, for example.
For the most part, installation technologies take care of a lot of things for you. However, there are some things very specific to your installation that you may want to verify for each edition of your MSI. While copying files and setting registry values work in Windows Installer, you may want to verify that all of the files you want copied get copied during an install, and that all of the registry values you want to set are set during an install.
As with any testing, coming up with a plan and prioritizing what to test often comes with understanding what the user wants out of the installation. Installations can become complex very...