test: Add dragonfly to test

Author: woodruffw

test/test_unistd.rs

@@ -1052,6 +1052,7 @@ fn test_ttyname_invalid_fd() {
     target_os = "freebsd",
     target_os = "openbsd",
     target_os = "netbsd",
+    target_os = "dragonfly",
 ))]
 fn test_getpeereid() {
     use std::os::unix::net::UnixStream;