Add test for cpu affinity

Add a test to check BIND 9 honors CPU affinity mask. This requires
some changes to the start script, to construct the named command.

bin/tests/system/cpu/prereq.sh

@@ -0,0 +1,30 @@
+#!/bin/sh -e
+#
+# 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.
+
+set -e
+
+# shellcheck source=conf.sh
+. ../conf.sh
+
+case $(uname) in
+	Linux*)
+		;;
+	*)
+		echo_i "cpu test only runs on Linux, skipping test"
+		exit 255
+		;;
+esac
+
+# TASKSET will be an empty string if no taskset program was found.
+TASKSET=$(command -v "taskset" || true)
+if ! test -x "$TASKSET" ; then
+	exit 255
+fi