Pull request: 2846 cover aghnet vol.4

Merge in DNS/adguard-home from 2846-cover-aghnet-vol.4 to master

Updates #2846.

Squashed commit of the following:

commit 576ef857628a403ce1478c10a4aad23985c09613
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Mar 31 19:38:57 2022 +0300

    aghnet: imp code

commit 5b4b17ff52867aaab2c9d30a0fc7fc2fe31ff4d5
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Mar 31 14:58:34 2022 +0300

    aghnet: imp coverage

internal/aghnet/arpdb_test.go

@@ -181,6 +181,16 @@ func TestCmdARPDB_arpa(t *testing.T) {
 		err := a.Refresh()
 		testutil.AssertErrorMsg(t, "cmd arpdb: running command: unexpected exit code 1", err)
 	})
+
+	t.Run("empty", func(t *testing.T) {
+		sh := theOnlyCmd("cmd", 0, "", nil)
+		substShell(t, sh.RunCmd)
+
+		err := a.Refresh()
+		require.NoError(t, err)
+
+		assert.Empty(t, a.Neighbors())
+	})
 }
 
 func TestEmptyARPDB(t *testing.T) {