posture: add get serial support for macOS

Updates #5902

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

posture/serialnumber_macos_test.go

@@ -0,0 +1,37 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build cgo && darwin && !ios
+
+package posture
+
+import (
+	"fmt"
+	"testing"
+
+	"tailscale.com/types/logger"
+	"tailscale.com/util/cibuild"
+)
+
+func TestGetSerialNumberMac(t *testing.T) {
+	// Do not run this test on CI, it can only be ran on macOS
+	// and we currenty only use Linux runners.
+	if cibuild.On() {
+		t.Skip()
+	}
+
+	sns, err := GetSerialNumbers(logger.Discard)
+	if err != nil {
+		t.Fatalf("failed to get serial number: %s", err)
+	}
+
+	if len(sns) != 1 {
+		t.Errorf("expected list of one serial number, got %v", sns)
+	}
+
+	if len(sns[0]) <= 0 {
+		t.Errorf("expected a serial number with more than zero characters, got %s", sns[0])
+	}
+
+	fmt.Printf("serials: %v\n", sns)
+}