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>

t/t9700/test.pl

@@ -38,7 +38,7 @@
 # Failure cases for config:
 # Save and restore STDERR; we will probably extract this into a
 # "dies_ok" method and possibly move the STDERR handling to Git.pm.
-open our $tmpstderr, ">&", STDERR or die "cannot save STDERR"; close STDERR;
+open our $tmpstderr, ">&STDERR" or die "cannot save STDERR"; close STDERR;
 eval { $r->config("test.dupstring") };
 ok($@, "config: duplicate entry in scalar context fails");
 eval { $r->config_bool("test.boolother") };