Fix testsuite for ./configure --enable-gcc-warnings:

* configure.ac (gcc-warnings): Move -Wall before -Wno-sign-compare.
Otherwise, gcc 4.1.0 (at least) warns about sign comparisons in
__AT_CHECK_PRINTER_AND_DESTRUCTOR in tests/actions.at.
* test/input.at (Torturing the Scanner): #include <stdlib.h> for abort.
* test/regression.at (Diagnostic that expects two alternatives):
Likewise.

tests/regression.at

@@ -959,6 +959,7 @@ m4_define([_AT_DATA_EXPECT2_Y],
 static int yylex (AT_LALR1_CC_IF([int *], [void]));
 AT_LALR1_CC_IF([],
 [#include <stdio.h>
+#include <stdlib.h>
 static void yyerror (const char *);])
 %}
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