t9700/test.pl: avoid bareword 'STDERR' in 3-argument open()

Some versions of perl complain when 'STDERR' is used as the third argument
in the 3-argument form of open(). Convert to the 2-argument form which is
described for duping STDERR in my second edition camel book.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Tested-by: Tom G. Christensen <tgc@statsbiblioteket.dk> on RHEL 3, Perl 5.8.0
Signed-off-by: Junio C Hamano <gitster@pobox.com>