test-skeleton: Support temporary files without memory leaks [BZ#18333]

add_temp_file now makes a copy which is freed by delete_temp_files.
Callers to create_temp_file can now free the returned file name to
avoid the memory leak.  These changes do not affect the leak behavior
of existing code.

Also address a NULL pointer derefence in tzset after a memoru allocation
failure, found during testing.

time/tzset.c

@@ -201,7 +201,12 @@ parse_tzname (const char **tzp, int whichrule)
       if (*p++ != '>' || len < 3)
 	return false;
     }
-  tz_rules[whichrule].name = __tzstring_len (start, len);
+
+  const char *name = __tzstring_len (start, len);
+  if (name == NULL)
+    return false;
+  tz_rules[whichrule].name = name;
+
   *tzp = p;
   return true;
 }