Pull request: all: imp ipset, add tests

Closes #2611.

Squashed commit of the following:

commit f72577757e5cd0299863ccc01780ad9307adc6ea
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jun 18 17:32:41 2021 +0300

    dnsforward: imp err msgs

commit ed8d6dd4b5d7171dfbd57742b53bf96be6cbec29
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jun 18 17:00:49 2021 +0300

    all: imp ipset code

commit 887b3268bae496f4ad616b277f5e28d3ee24a370
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jun 18 16:09:08 2021 +0300

    dnsforward: imp ipset, add tests

internal/dnsforward/ipset_test.go

@@ -0,0 +1,116 @@
+package dnsforward
+
+import (
+	"net"
+	"testing"
+
+	"github.com/AdguardTeam/dnsproxy/proxy"
+	"github.com/miekg/dns"
+	"github.com/stretchr/testify/assert"
+)
+
+// fakeIpsetMgr is a fake aghnet.IpsetManager for tests.
+type fakeIpsetMgr struct {
+	ip4s []net.IP
+	ip6s []net.IP
+}
+
+// Add implements the aghnet.IpsetManager inteface for *fakeIpsetMgr.
+func (m *fakeIpsetMgr) Add(host string, ip4s, ip6s []net.IP) (n int, err error) {
+	m.ip4s = append(m.ip4s, ip4s...)
+	m.ip6s = append(m.ip6s, ip6s...)
+
+	return len(ip4s) + len(ip6s), nil
+}
+
+// Close implements the aghnet.IpsetManager interface for *fakeIpsetMgr.
+func (*fakeIpsetMgr) Close() (err error) {
+	return nil
+}
+
+func TestIpsetCtx_process(t *testing.T) {
+	ip4 := net.IP{1, 2, 3, 4}
+	ip6 := net.IP{
+		0x12, 0x34, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x56, 0x78,
+	}
+
+	req4 := createTestMessageWithType("example.com", dns.TypeA)
+	req6 := createTestMessageWithType("example.com", dns.TypeAAAA)
+
+	resp4 := &dns.Msg{
+		Answer: []dns.RR{&dns.A{
+			A: ip4,
+		}},
+	}
+	resp6 := &dns.Msg{
+		Answer: []dns.RR{&dns.AAAA{
+			AAAA: ip6,
+		}},
+	}
+
+	t.Run("nil", func(t *testing.T) {
+		dctx := &dnsContext{
+			proxyCtx: &proxy.DNSContext{},
+
+			responseFromUpstream: true,
+		}
+
+		ictx := &ipsetCtx{}
+		rc := ictx.process(dctx)
+		assert.Equal(t, resultCodeSuccess, rc)
+
+		err := ictx.close()
+		assert.NoError(t, err)
+	})
+
+	t.Run("ipv4", func(t *testing.T) {
+		dctx := &dnsContext{
+			proxyCtx: &proxy.DNSContext{
+				Req: req4,
+				Res: resp4,
+			},
+
+			responseFromUpstream: true,
+		}
+
+		m := &fakeIpsetMgr{}
+		ictx := &ipsetCtx{
+			ipsetMgr: m,
+		}
+
+		rc := ictx.process(dctx)
+		assert.Equal(t, resultCodeSuccess, rc)
+		assert.Equal(t, []net.IP{ip4}, m.ip4s)
+		assert.Empty(t, m.ip6s)
+
+		err := ictx.close()
+		assert.NoError(t, err)
+	})
+
+	t.Run("ipv6", func(t *testing.T) {
+		dctx := &dnsContext{
+			proxyCtx: &proxy.DNSContext{
+				Req: req6,
+				Res: resp6,
+			},
+
+			responseFromUpstream: true,
+		}
+
+		m := &fakeIpsetMgr{}
+		ictx := &ipsetCtx{
+			ipsetMgr: m,
+		}
+
+		rc := ictx.process(dctx)
+		assert.Equal(t, resultCodeSuccess, rc)
+		assert.Empty(t, m.ip4s)
+		assert.Equal(t, []net.IP{ip6}, m.ip6s)
+
+		err := ictx.close()
+		assert.NoError(t, err)
+	})
+}