You do not need a special FON (FinanzOnline) account in the TEST environment of the SIGN AT API (RKSV), because the communication between FON and fiskaly is only simulated here.
Any data can be used in the fields fon_participant_id, fon_user_id, fon_user_pin in the TEST environment, these will not be validated against FON (FinanzOnline).
Please also read our article about the differences between the LIVE and the TEST environment.