tests: check $$'s destruction with variant, YYERROR, and no error recovery

When variant are enabled, the yylhs variable (the left-hand side of
the rule being reduced, i.e. $$ and @$) is explicitly destroyed when
YYERROR is called.  This is because before running the user code, $$
is initialized, so that the user can properly use it.

However, when quitting yyparse, yylhs is also reclaimed by the C++
compiler: the variable goes out of scope.

This was not detected by the test suite because (i) the Object tracker
was too weak, and (ii) the problem does not show when there is error
recovery.

Reported by Paolo Simone Gasparello.
<http://lists.gnu.org/archive/html/bug-bison/2013-10/msg00003.html>

* tests/c++.at (Exception safety): Improve the objects logger to make
sure that we never destroy twice an object.
Also track copy-constructors.
Use a set instead of a list.
Display the logs before running the function body, this is more
useful in case of failure.
Generalize to track with and without error recovery.

THANKS

@@ -96,6 +96,7 @@ Odd Arild Olsen           oao@fibula.no
 Oleg Smolsky              oleg.smolsky@pacific-simulators.co.nz
 Oleksii Taran             oleksii.taran@gmail.com
 Paolo Bonzini             bonzini@gnu.org
+Paolo Simone Gasparello   djgaspa@gmail.com
 Pascal Bart               pascal.bart@epita.fr
 Paul Eggert               eggert@cs.ucla.edu
 Paul Hilfinger            Hilfinger@CS.Berkeley.EDU