tests: use %empty instead of comments

* tests/c++.at, tests/glr-regression.at: here.

tests/glr-regression.at

@@ -591,7 +591,7 @@ AT_DATA_GRAMMAR([glr-regr7.y],
 start:
     stack1 start
   | stack2 start
-  | /* empty */
+  | %empty
   ;
 stack1: 'a' ;
 stack2: 'a' ;
@@ -680,11 +680,11 @@ InterfaceDeclaration    : OptConstantWord       %dprec 1
         | OptSignalWord %dprec 2
         ;
 
-OptConstantWord : /* empty */
+OptConstantWord : %empty
         | T_CONSTANT
         ;
 
-OptSignalWord   : /* empty */
+OptSignalWord   : %empty
                 { printf("empty: %d/%d\n", @$.first_column, @$.last_column); }
         | T_SIGNAL
         ;