tests: be robust to -DNDEBUG

input.y: In function 'yylex':
input.y:67:7: error: unused variable 'input_elts' [-Werror=unused-variable]
   int input_elts = sizeof input / sizeof input[0];
       ^~~~~~~~~~
cc1: all warnings being treated as errors

* tests/input.at, tests/local.at: Avoid that.

tests/input.at

@@ -1321,6 +1321,7 @@ yylex (void)
 #output "; /* "
   */
   enum { input_elts = sizeof input };
+  (void) input_elts;
   static ptrdiff_t toknum;
   assert (0 <= toknum && toknum < input_elts);
   yylval = value_as_yystype (input[toknum]);

tests/local.at

@@ -559,6 +559,7 @@ static
   int res;
   ]AT_USE_LEX_ARGS[
   int input_elts = sizeof input / sizeof input[0];
+  (void) input_elts;
   assert (0 <= toknum && toknum < input_elts);
   res = input[toknum++];
   ]$2[;]AT_TOKEN_CTOR_IF([], [[