tests: factor the declaration/definition of yyerror and yylex.

* tests/local.at (AT_YYERROR_DECLARE, AT_YYERROR_DECLARE_EXTERN)
(AT_YYERROR_DEFINE, AT_YYLEX_DECLARE, AT_YYLEX_DECLARE_EXTERN)
(AT_YYLEX_DEFINE): New.
Must be used inside AT_BISON_OPTION_PUSHDEFS/POPDEFS pair.
* tests/actions.at, tests/conflicts.at, tests/glr-regression.at,
* tests/headers.at, tests/input.at, tests/named-refs.at,
* tests/regression.at, tests/skeletons.at, tests/synclines.at,
* tests/torture.at: Use them.