syscall: re-enable TestPassFD on dragonfly

At least it works well on DragonFly BSD 4.6.

Change-Id: I3b210745246c6d8d42e32ba65ee3b9a17d171ff7
Reviewed-on: https://go-review.googlesource.com/30170
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_unix_test.go

@@ -126,9 +126,6 @@ func TestFcntlFlock(t *testing.T) {
 // that the test should become the child process instead.
 func TestPassFD(t *testing.T) {
 	switch runtime.GOOS {
-	case "dragonfly":
-		// TODO(jsing): Figure out why sendmsg is returning EINVAL.
-		t.Skip("skipping test on dragonfly")
 	case "solaris":
 		// TODO(aram): Figure out why ReadMsgUnix is returning empty message.
 		t.Skip("skipping test on solaris, see issue 7402")