t/op/reverse.t: add a test for single character reversal

This could help detect implementation bugs in algorithms for reverse
copying from a source buffer to a different destination buffer.

Author: richardleach

t/op/reverse.t

@@ -6,7 +6,9 @@ BEGIN {
     set_up_inc('../lib');
 }
 
-plan tests => 25;
+plan tests => 26;
+
+is(reverse("a"), "a", 'single char reverse');
 
 is(reverse("abc"), "cba", 'simple reverse');