Hey Nicolas,
The best way to test a "clean slate" of a computer is to use a fully-updated virtual machine, make a snapshot of it, then run whatever tests you need to run. Then, when you need to re-run tests, you can certainly do that by going back to that snapshot and re-running the tests.
Some data can be "cleaned completely", for example the activation and product key data can be removed completely using TA_Deactivate(your_handle, 1);
Some data cannot be "cleaned completely" (for example, verified trial data, while the files can be removed, that doesn't actually re-set the trial because the fingerprint of that computer is stored on the activation servers and when you "restart" the trial on the snapshot you'll resume where you left off on the trial).