tests: improve the language independance layer

* tests/local.at (_AT_LANG_DISPATCH): New, shamelessly stolen from
Autoconf's _AT_LANG_DISPATCH.
(AT_LANG_DISPATCH): New.
(AT_YYERROR_FORMALS, AT_YYERROR_PROTOTYPE, AT_YYERROR_DECLARE_EXTERN)
(AT_YYERROR_DECLARE, AT_YYERROR_DEFINE, AT_MAIN_DEFINE, AT_COMPILE)
(AT_FULL_COMPILE):
Use AT_LANG_DISPATCH instead of an ad hoc m4_case.