tests: highlight empty right-hand sides

* tests/actions.at, tests/c++.at, tests/headers.at,
* tests/input.at: here.

tests/headers.at

@@ -41,7 +41,7 @@ AT_DATA_GRAMMAR([$1.y],
 ]AT_YYLEX_DECLARE_EXTERN[
 %}
 %%
-dummy:;
+dummy: /* empty */;
 %%
 #include <$1.h>
 ])
@@ -81,7 +81,7 @@ AT_DATA_GRAMMAR([input.y],
 ]AT_YYLEX_DEFINE[
 %}
 %%
-exp:;
+exp: /* empty */;
 ]])
 
 AT_BISON_CHECK([--defines -o input.c input.y])