tests: use %empty instead of comments

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

tests/c++.at

@@ -893,7 +893,7 @@ error-reduce:
 
 consistent-error:
   'a'
-| /*empty*/ %prec 'a'
+| %empty %prec 'a'
 ;
 
 // Provide another context in which all rules are useful so that this

tests/conflicts.at

@@ -278,17 +278,17 @@ AT_DATA([[input.y]],
 [[%expect 1
 %%
 exp: e 'e';
-e: 'e' | /* Nothing. */;
+e: 'e' | %empty;
 ]])
 
 AT_BISON_CHECK([-o input.c input.y], 0, [],
-[[input.y:4.9: warning: rule useless in parser due to conflicts [-Wother]
+[[input.y:4.10-15: warning: rule useless in parser due to conflicts [-Wother]
 ]])
 
 AT_BISON_CHECK([-fcaret -o input.c input.y], 0, [],
-[[input.y:4.9: warning: rule useless in parser due to conflicts [-Wother]
-    4 | e: 'e' | /* Nothing. */;
-      |         ^
+[[input.y:4.10-15: warning: rule useless in parser due to conflicts [-Wother]
+    4 | e: 'e' | %empty;
+      |          ^~~~~~
 ]])
 
 AT_CLEANUP

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
         ;