Fix stability confusion in sort-tests

Problem reported by Philipp Stephani (Bug#23205).
* test/automated/sort-tests.el:
(sort-tests--insert-words-sort-and-compare):
Don’t assume that reversing a sorted list is the same
as sorting with the reverse predicate.  This is not true
for stable sorts when items compare equal.

test/automated/sort-tests.el

@@ -40,8 +40,10 @@
     (funcall function reverse (point-min) (point-max))
     (let ((sorted-words
            (mapconcat #'identity
-                      (let ((x (sort (copy-sequence words) less-predicate)))
-                        (if reverse (reverse x) x))
+                      (sort (copy-sequence words)
+                            (if reverse
+                                (lambda (a b) (funcall less-predicate b a))
+                              less-predicate))
                       separator)))
       (should (string= (substring (buffer-string) 0 -1) sorted-words)))))