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/ns1/example.db

@@ -0,0 +1,23 @@
+; 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 http://mozilla.org/MPL/2.0/.
+;
+; See the COPYRIGHT file distributed with this work for additional
+; information regarding copyright ownership.
+
+$TTL 300	; 5 minutes
+@			SOA	mname1. . (
+				2000062101 ; serial
+				20         ; refresh (20 seconds)
+				20         ; retry (20 seconds)
+				1814400    ; expire (3 weeks)
+				3600       ; minimum (1 hour)
+				)
+			NS	ns1
+ns1			A	10.53.0.1
+@			A	10.53.0.1
+a			A	10.53.0.1
+b			A	10.53.0.1
+$INCLUDE large.db