tests: enable -Wsign-compare and fix corresponding warnings

-Wsign-compare was disabled for bison's own code, following gnulib's
approach.  However, the generated parsers should not trigger such
warnings.

Reported by Efi Fogel.
http://lists.gnu.org/archive/html/help-bison/2013-04/msg00018.html

See also http://stackoverflow.com/questions/16101062 for the weird
"-(unsigned)i" piece of code.

* configure.ac (warn_tests): Enable -Wsign-compare.
* data/location.cc (position::add_): New.
(position::lines, position::columns): Use it.
* tests/actions.at (AT_CHECK_PRINTER_AND_DESTRUCTOR): Fix signedness issues.

THANKS

@@ -38,6 +38,7 @@ Derek M. Jones            derek@knosof.co.uk
 Di-an Jan                 dianj@freeshell.org
 Dick Streefland           dick.streefland@altium.nl
 Didier Godefroy           dg@ulysium.net
+Efi Fogel                 efifogel@gmail.com
 Enrico Scholz             enrico.scholz@informatik.tu-chemnitz.de
 Eric Blake                ebb9@byu.net
 Evgeny Stambulchik        fnevgeny@plasma-gate.weizmann.ac.il