Create Unit tests for backend

[leticiaarj]

Perform unit tests on the backend project, having code coverage and possible error analysis.

Tasks

• [ ] Using PyUnit for testing.
• [ ] Test as many methods as possible.
• [ ] Check the return of each test and signal the team about possible errors.

Acceptance criteria

• [ ] The tests are working.
• [ ] The project is running.

[leticiaarj]

Derived from the issue #146.