Add checkonf tests for the PROXYv2 related options

This commit adds necessary PROXYv2 configuration options checks.

bin/tests/system/checkconf/bad-proxy-encrypted-doh-plain.conf

@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0.  If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+http local-http-server {
+	endpoints { "/dns-query"; };
+	listener-clients 100;
+	streams-per-connection 100;
+};
+
+options {
+	listen-on port 8080 proxy encrypted tls none http local-http-server { 10.53.0.1; };
+};