I went mechanical for testing this. We had an app which allowed you to log in with a fingerprint for 24 hours if you had recently logged in with a password.
To test the app, I created a latex mold of the bottom of my foot (didn’t want to use a real fingerprint). I then used that mold to register it as my fingerprint. Once that was registered with the iPhone I could tape it to the fingerprint sensor. Our app was designed so it would start up then check for the fingerprint. So we could just leave it taped to the sensor and it seemed to work.
I was planning on using Lego Mindstorm to build a robot that I could control to tap the sensor with the fingerprint but it turned out I did not need that.