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/dhcpd/options_unix_test.go

@@ -95,6 +95,7 @@ func TestParseOpt(t *testing.T) {
 			opt, err := parseDHCPOption(tc.in)
 			if tc.wantErrMsg != "" {
 				require.Error(t, err)
+
 				assert.Equal(t, tc.wantErrMsg, err.Error())
 
 				return