net: mark TestDialerDualStackFDLeak as flaky on OpenBSD

Updates #15157

Change-Id: Id280705f4382c3b2323f0eed786a400a184614de
Reviewed-on: https://go-review.googlesource.com/34656Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dial_test.go

@@ -142,6 +142,8 @@ func TestDialerDualStackFDLeak(t *testing.T) {
 		t.Skipf("%s does not have full support of socktest", runtime.GOOS)
 	case "windows":
 		t.Skipf("not implemented a way to cancel dial racers in TCP SYN-SENT state on %s", runtime.GOOS)
+	case "openbsd":
+		testenv.SkipFlaky(t, 15157)
 	}
 	if !supportsIPv4 || !supportsIPv6 {
 		t.Skip("both IPv4 and IPv6 are required")