posture: add get serial support for Windows/Linux

This commit adds support for getting serial numbers from SMBIOS
on Windows/Linux (and BSD) using go-smbios.

Updates #5902

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

posture/serialnumber_notmacos_test.go

@@ -0,0 +1,38 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Build on Windows, Linux and *BSD
+
+//go:build windows || (linux && !android) || freebsd || openbsd || dragonfly || netbsd
+
+package posture
+
+import (
+	"fmt"
+	"testing"
+
+	"tailscale.com/types/logger"
+)
+
+func TestGetSerialNumberNotMac(t *testing.T) {
+	// This test is intentionally skipped as it will
+	// require root on Linux to get access to the serials.
+	// The test case is intended for local testing.
+	// Comment out skip for local testing.
+	t.Skip()
+
+	sns, err := GetSerialNumbers(logger.Discard)
+	if err != nil {
+		t.Fatalf("failed to get serial number: %s", err)
+	}
+
+	if len(sns) == 0 {
+		t.Fatalf("expected at least 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)
+}