cherry-pick: 2846 cover aghnet vol.1

Merge in DNS/adguard-home from 2846-cover-aghnet-vol.1 to master

Updates #2846.

Squashed commit of the following:

commit 368e75b0bacb290f9929b8a5a682b06f2d75df6a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jan 21 19:11:59 2022 +0300

    aghnet: imp tests

commit 8bb3e2a1680fd30294f7c82693891ffb19474c6a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jan 21 18:27:06 2022 +0300

    aghnet: rm unused test

commit 28d8e64880f845810d0af629e5d1f06b9bde5b28
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jan 21 18:18:22 2022 +0300

    aghnet: cover with tests

internal/aghnet/ipmut_test.go

@@ -0,0 +1,44 @@
+package aghnet
+
+import (
+	"net"
+	"testing"
+
+	"github.com/AdguardTeam/golibs/netutil"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIPMut(t *testing.T) {
+	testIPs := []net.IP{{
+		127, 0, 0, 1,
+	}, {
+		192, 168, 0, 1,
+	}, {
+		8, 8, 8, 8,
+	}}
+
+	t.Run("nil_no_mut", func(t *testing.T) {
+		ipmut := NewIPMut(nil)
+
+		ips := netutil.CloneIPs(testIPs)
+		for i := range ips {
+			ipmut.Load()(ips[i])
+			assert.True(t, ips[i].Equal(testIPs[i]))
+		}
+	})
+
+	t.Run("not_nil_mut", func(t *testing.T) {
+		ipmut := NewIPMut(func(ip net.IP) {
+			for i := range ip {
+				ip[i] = 0
+			}
+		})
+		want := netutil.IPv4Zero()
+
+		ips := netutil.CloneIPs(testIPs)
+		for i := range ips {
+			ipmut.Load()(ips[i])
+			assert.True(t, ips[i].Equal(want))
+		}
+	})
+}