TASK Registration unit tests

[roosavihlman]

Under this ticket is created unit tests for registration backend implementation. Backend is implemented under ticket #6.

DoD: Unit tests created for:

• registration attempt for email that already has an account
• registration attempt for username already existing
• successful registration (user saved to database)
• email sent successfully
• failed email sending

[roosavihlman]

PR: https://github.com/APUOPE-RE/backend/pull/18

Created unit tests described above, also updated db usage in unit tests and removed some unnecessary prints from emailService.