Update simple_linked_list_test.go with an idempotency test

Programming Go
1

If the submitter changes the actual list while iterating through its elements, the existing tests won’t catch that. Example of a String() method that changes the underlying list:

func (l *List) String() string {
var listString string
for l.head != nil {
listString += fmt.Sprintf("%d ", l.head.value)
l.head = l.head.next
}
return listString
}
2

The function that just traverses the list in the exercise is the Array function. Are you suggesting that the test for the Array function should check if the list is not changed?