tests: pacify gcc 4.6.0's -Wunused-but-set-variable.

Reported by Jim Meyering at
<http://lists.gnu.org/archive/html/bug-bison/2011-04/msg00002.html>.
* tests/actions.at
(Default %printer and %destructor for mid-rule values): Define
YYLLOC_DEFAULT so that it uses its Rhs argument.
(cherry picked from commit c9e2da4f20)

tests/actions.at

@@ -1199,7 +1199,7 @@ AT_DATA_GRAMMAR([[input.y]],
   static int yylex (void);
 # define USE(SYM)
 # define YYLTYPE int
-# define YYLLOC_DEFAULT(Current, Rhs, N)
+# define YYLLOC_DEFAULT(Current, Rhs, N) (void)(Rhs)
 # define YY_LOCATION_PRINT(File, Loc)
 %}