Added Test for AddBook Method
  Testing the AddBook method has multple test functions
      1. testAddBook()
         To test if book added successfully
      2. testAddDuplicateBook()
         Should not add the book with same ISBN but it adds
      3. testAddNegativeISBNBook()
         Should not add the book with negative value of ISBN but it adds
      4. testAddCharISBNBook()
         Should not add the book with character value of ISBN but it adds