tests: factor the definition of main

With Théophile Ranquet.

* tests/local.at (AT_MAIN_DEFINE): New.
(AT_YYERROR_DEFINE): Improve formatting.
* tests/actions.at, tests/c++.at, tests/conflicts.at,
* tests/glr-regression.at, tests/input.at, tests/regression.at,
* tests/skeletons.at, tests/torture.at: Adjust.
* tests/c++.at: Add missing %skeleton for a PUSHDEFS, and a missing
PUSH/POPDEFS for another test.

tests/input.at

@@ -859,12 +859,7 @@ AT_DATA([main.c],
 #include "input.h"
 
 int yyparse (void);
-
-int
-main (void)
-{
-  return yyparse ();
-}
+]AT_MAIN_DEFINE[
 ]])
 AT_BISON_OPTION_POPDEFS