Make t/pty.t test pass on OpenBSD.

Index: t/pty.t
--- t/pty.t.orig
+++ t/pty.t
@@ -141,6 +141,10 @@ SKIP: {
     is( _map_fds, $fd_map );
 }
 
+# OpenBSD libc devname(3) opens /var/run/dev.db and keeps it open.
+# As this would confuse open file descriptor checks, open it in advance.
+IO::Pty->new();
+
 ##
 ## stdout, stderr
 ##