testlib/OpenSSL/Test.pm: keep default input private.

If $_ is not private, it can wipe caller's one, which proved to be
problematic...
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

test/testlib/OpenSSL/Test.pm

@@ -460,6 +460,7 @@ sub run {
     # to make it easier to compare with a manual run of the command.
     if ($opts{capture} || defined($opts{prefix})) {
 	my $pipe;
+	local $_;
 
 	open($pipe, '-|', "$prefix$cmd") or die "Can't start command: $!";
 	while(<$pipe>) {