Add authentication controller & model tests

[macropusgiganteus]

What happened 👀

Implemented controller test and model test

• User model test
• User controller test
• UserSessions controller test

Insight 📝

Implemented test using Rails Minitest (ref: Testing Rails Applications, 3.3 Getting started with testing)

• Controller: users controller, user_sessions controller
• Model: users model

Proof Of Work 📹