add basic DoH system tests

- rename dot to doth, as it now covers both dot and doh.
- merge xot into doth as it's closely related.
- added long-lived key and cert files (expiring 2121).
- add tests with https-get, https-post, http-plain, alternate
  endpoints, and both static and ephemeral TLS configuration.
- incidentally fixed a memory leak in dig that occurred if +https
  was specified more than once.

bin/tests/system/doth/setup.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+#
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# 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.
+
+. ../conf.sh
+
+$SHELL ${TOP_SRCDIR}/bin/tests/system/genzone.sh 2 >ns1/example.db
+
+copy_setports ns1/named.conf.in ns1/named.conf
+copy_setports ns2/named.conf.in ns2/named.conf