Update simple_linked_list_test.go with an idempotency test

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
}