Pull request: use testutil

Squashed commit of the following:

commit 5345a14b3565f358c56a37500cafb35b7e397951
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 21 21:13:06 2021 +0300

    all: fix windows tests

commit 8b9cdbe3e78f43339d21277f04e686bb154f6968
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 21 20:23:55 2021 +0300

    all: imp code

commit 271fdbe74c29d8ea4b53d7f56d2a36612dfed7b3
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 21 19:43:32 2021 +0300

    all: imp testing

commit e340f9d48679c57fc8eb579b8b78d4957be111c4
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 21 18:53:51 2021 +0300

    all: use testutil

internal/aghnet/systemresolvers_others_test.go

@@ -79,8 +79,8 @@ func TestSystemResolvers_DialFunc(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			conn, err := imp.dialFunc(context.Background(), "", tc.address)
-
 			require.Nil(t, conn)
+
 			assert.ErrorIs(t, err, tc.want)
 		})
 	}