Pull request: 2574 external tests vol.3

Merge in DNS/adguard-home from 2574-external-tests-3 to master

Updates #2574.

Squashed commit of the following:

commit 29d429c65dee2621ca503710a7ba9522f14f55f9
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Feb 4 20:06:57 2021 +0300

    all: finally fix spacing

commit 9e3a3be63b74852a7802e3f1832648444b58e4d0
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Feb 4 19:59:09 2021 +0300

    aghtest: polish spacing

commit 8a984159fe813b95b989803f5b8b78d01a41bd39
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Feb 4 18:44:47 2021 +0300

    all: fix linux tests, imp code quality

commit 0c1b42bacba1b23fa847e1fa032579c525b3eaa1
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Feb 4 17:33:12 2021 +0300

    all: mv testutil to aghtest package, imp tests

internal/aghtest/aghtest.go

@@ -0,0 +1,47 @@
+// Package aghtest contains utilities for testing.
+package aghtest
+
+import (
+	"io"
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/AdguardTeam/golibs/log"
+)
+
+// DiscardLogOutput runs tests with discarded logger output.
+func DiscardLogOutput(m *testing.M) {
+	// TODO(e.burkov): Refactor code and tests to not use the global mutable
+	// logger.
+	log.SetOutput(ioutil.Discard)
+
+	os.Exit(m.Run())
+}
+
+// ReplaceLogWriter moves logger output to w and uses Cleanup method of t to
+// revert changes.
+func ReplaceLogWriter(t *testing.T, w io.Writer) {
+	stdWriter := log.Writer()
+	t.Cleanup(func() {
+		log.SetOutput(stdWriter)
+	})
+	log.SetOutput(w)
+}
+
+// ReplaceLogLevel sets logging level to l and uses Cleanup method of t to
+// revert changes.
+func ReplaceLogLevel(t *testing.T, l int) {
+	switch l {
+	case log.INFO, log.DEBUG, log.ERROR:
+		// Go on.
+	default:
+		t.Fatalf("wrong l value (must be one of %v, %v, %v)", log.INFO, log.DEBUG, log.ERROR)
+	}
+
+	stdLevel := log.GetLevel()
+	t.Cleanup(func() {
+		log.SetLevel(stdLevel)
+	})
+	log.SetLevel(l)
+}