Pull request: 2704 local resolvers vol.1

Merge in DNS/adguard-home from 2704-local-addresses-vol.1 to master

Updates #2704.
Updates #2829.
Updates #2846.

Squashed commit of the following:

commit 9a49b3d27edcb30da7f16a065226907833b1dc81
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Mar 22 15:39:17 2021 +0300

    aghnet: imp docs and logging

commit 74f95a29c55b9e732276601b0ecc63fb7c3a9f9e
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 20:56:51 2021 +0300

    all: fix friday evening mistakes

commit 0e2066bc5c16ed807fa601780b99e154502361a9
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 20:51:15 2021 +0300

    all: upd testify, imp code quality

commit 8237c50b670c58361ccf7adec3ff2452b1196677
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 20:19:29 2021 +0300

    aghnet: imp test naming

commit 14eb1e189339554c0a6d38e2ba7a93917774ebab
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 19:41:43 2021 +0300

    aghnet: isolate windows-specific functionality

commit d461ac8b18c187999da3e3aba116571b7ebe6785
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 14:50:05 2021 +0300

    aghnet: imp code quality

commit d0ee01cb1f8613de2085c0f2f2f396e46beb52a5
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 11:59:10 2021 +0300

    all: mv funcs to agherr, mk system resolvers getter

internal/aghnet/systemresolvers_test.go

@@ -0,0 +1,33 @@
+package aghnet
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func createTestSystemResolvers(
+	t *testing.T,
+	refreshDur time.Duration,
+	hostGenFunc HostGenFunc,
+) (sr SystemResolvers) {
+	t.Helper()
+
+	var err error
+	sr, err = NewSystemResolvers(refreshDur, hostGenFunc)
+	require.NoError(t, err)
+	require.NotNil(t, sr)
+
+	return sr
+}
+
+func TestSystemResolvers_Get(t *testing.T) {
+	sr := createTestSystemResolvers(t, 0, nil)
+	assert.NotEmpty(t, sr.Get())
+}
+
+// TODO(e.burkov): Write tests for refreshWithTicker.
+//
+// See https://github.com/AdguardTeam/AdGuardHome/issues/2846.