Just for your information, some testcases was already written on
http://testcases.qa.ubuntu.com/Applications (see LXDE / Lubuntu).

I also have several ideas of testcases that could be useful for us,
maybe only run once when doing the test of the ISO :
- Testing with a system with low RAM
- Testing with a system with low space available
- Testing update the language support post-installation
- ...