Add TCP timeouts system test

The system tests were missing a test that would test tcp-initial-timeout
and tcp-idle-timeout.

This commit adds new "timeouts" system test that adds:

  * Test that waits longer than tcp-initial-timeout and then checks
    whether the socket was closed

  * Test that sends and receives DNS message then waits longer than
    tcp-initial-timeout but shorter time than tcp-idle-timeout than
    sends DNS message again than waits longer than tcp-idle-timeout
    and checks whether the socket was closed

  * Similar test, but bursting 25 DNS messages than waiting longer than
    tcp-initial-timeout and shorter than tcp-idle-timeout than do second
    25 DNS message burst

  * Check whether transfer longer than tcp-initial-timeout succeeds

bin/tests/system/timeouts/prereq.sh

@@ -0,0 +1,29 @@
+#!/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.
+
+SYSTEMTESTTOP=..
+. $SYSTEMTESTTOP/conf.sh
+
+if test -n "$PYTHON"
+then
+    if $PYTHON -c "from dns.query import send_tcp" 2> /dev/null
+    then
+        :
+    else
+        echo_i "This test requires the dnspython >= 2.0.0 module." >&2
+        exit 1
+    fi
+else
+    echo_i "This test requires Python and the dnspython module." >&2
+    exit 1
+fi
+
+exit 0