Una implementación de la estructura de datos pila (stack).
Los casos de prueba están agrupadas por los métodos de la pila:
push_shouldAddObject_whenGivenValidObjectpush_shouldAddManyObjects_whenGivenValidValuespush_shouldThrowException_whenGivenNull
pop_shouldRemoveObject_whenStackIsNotEmpty📌pop_shouldReturnNull_whenStackIsEmpty
peek_shouldReturnLastObject_whenStackIsNotEmpty📌peek_shouldReturnNull_whenStackIsEmpty📌
size_shouldBeZero_whenStackIsInstantiated📌size_shouldIncrement_whenObjectIsPushedsize_shouldDecrement_whenObjectIsPopped