Skip the zero, xfer and ixfr tests on non-Linux platforms

Due to the platform differences, on non-Linux platforms, the xfer and
ixfr tests fails and zero test gets stuck.

This commit will get reverted when we add support for netmgr
multi-threading.

bin/tests/system/ixfr/prereq.sh

@@ -0,0 +1,19 @@
+#!/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
+
+if [ "$(uname -s)" != "Linux" ]; then
+    echo_i "This test is currently broken on non-Linux platforms"
+    exit 255
+fi
+
+exit 0