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/skeletons.at

@@ -106,12 +106,7 @@ yylex (void)
 {
   return 'a';
 }
-
-int
-main (void)
-{
-  return yyparse ();
-}
+]AT_MAIN_DEFINE[
 ]])
 
 AT_DATA_GRAMMAR([[input-cmd-line.y]],