Add unit tests for /delete route

[santosderek]

What would you like to add? Need to add unit tests for the newly added /delete route.

Describe the solution you'd like (If needed)

• [x] Unit tests must be in test_flask.py.
• [x] Must check if a non-signed in user gets redirected.
• [x] Must check if testTrainer can be deleted.
  • Use login_as_trainer() function to log in as a trainer.
• [x] Must check if testTrainee can be deleted.
  • Use login_as_trainee() function to log in as a trainee.
• [x] See if you can change the session['user_id'] to something that does not exist and be redirected to HTTP error 500 page.