Adding unit tests for Book and Label Class

[orozCoding]

In this branch, I created unit tests for the Book and Label class.

Book class tests

The book tests check if a Book is created properly and also check if the move_to_archived method works correctly.

Label class tests

The Label tests check if a Label is created properly and also check if the add_item method works correctly.

Closes #13

[orozCoding]

Thanks @TimmyChan99