You can do your FTP tests outside just fine, as long as you can find a stretch of road where you can ride for 20 minutes without having to stop for traffic all the time. It can also be a loop. An all-out 20 min effort will give you roughly 105% of FTP and as I stated above it is not an exact science and possibly you need to adjust FTP estimation if intervals are too hard/easy.
And also, you need to have those stretches of road with fairly interrupted riding anyway in order to do your regular interval workouts.

The value of the trainer is really so you can continue with your training program when weather outside is no good. Or to save time, as a 1-hour workout really just takes 1 hour, while outside there is much more overhead time waisted.