[1pt] Add unit tests for all implemented methods.

[JuanLPalacios]

Add unit tests for all implemented methods.

• add_item method in the Label class
  • should take an instance of the Item class as an input
  • should add the input item to the collection of items
  • should add self as a property of the item object (by using the correct setter from the item object)
• can_be_archived?() in the Book class
  • should override the method from the parent class
  • should return true if parent's method returns true OR if cover_state equals to "bad"
  • otherwise, it should return false