net/dns: fix IsZero and Equal methods on OSConfig

Discovered this while investigating the following issue; I think it's
unrelated, but might as well fix it. Also, add a test helper for
checking things that have an IsZero method using the reflect package.

Updates tailscale/support-escalations#55

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I57b7adde43bcef9483763b561da173b4c35f49e2

net/dns/osconfig_test.go

@@ -6,8 +6,10 @@ package dns
 import (
 	"fmt"
 	"net/netip"
+	"reflect"
 	"testing"
 
+	"tailscale.com/tstest"
 	"tailscale.com/util/dnsname"
 )
 
@@ -41,3 +43,13 @@ func TestOSConfigPrintable(t *testing.T) {
 		t.Errorf("format mismatch:\n   got: %s\n  want: %s", s, expected)
 	}
 }
+
+func TestIsZero(t *testing.T) {
+	tstest.CheckIsZero[OSConfig](t, map[reflect.Type]any{
+		reflect.TypeFor[dnsname.FQDN](): dnsname.FQDN("foo.bar."),
+		reflect.TypeFor[*HostEntry](): &HostEntry{
+			Addr:  netip.AddrFrom4([4]byte{100, 1, 2, 3}),
+			Hosts: []string{"foo", "bar"},
+		},
+	})
+}