[toranj] add new test to verify lowpan fragmentation (#4945)

This commit adds a new test to verify 6LoWPAN fragmentation logic by
exchanging IPv6 messages with many different lengths between two
nodes.

tests/toranj/start.sh

@@ -140,6 +140,7 @@ run test-037-wpantund-auto-add-route-for-on-mesh-prefix.py
 run test-038-clear-address-cache-for-sed.py
 run test-039-address-cache-table-snoop.py
 run test-040-network-data-stable-full.py
+run test-041-lowpan-fragmentation.py
 run test-100-mcu-power-state.py
 run test-600-channel-manager-properties.py
 run test-601-channel-manager-channel-change.py

tests/toranj/test-041-lowpan-fragmentation.py

@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+#
+#  Copyright (c) 2020, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Redistribution and use in source and binary forms, with or without
+#  modification, are permitted provided that the following conditions are met:
+#  1. Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+#  2. Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#  3. Neither the name of the copyright holder nor the
+#     names of its contributors may be used to endorse or promote products
+#     derived from this software without specific prior written permission.
+#
+#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#  POSSIBILITY OF SUCH DAMAGE.
+
+import wpan
+from wpan import verify
+
+# -----------------------------------------------------------------------------------------------------------------------
+# Test description: This test verifies 6LoWPAN fragmentation code by exchanging IPv6 messages with
+# many different lengths between two nodes.
+
+test_name = __file__[:-3] if __file__.endswith('.py') else __file__
+print('-' * 120)
+print('Starting \'{}\''.format(test_name))
+
+# -----------------------------------------------------------------------------------------------------------------------
+# Creating `wpan.Nodes` instances
+
+speedup = 4
+wpan.Node.set_time_speedup_factor(speedup)
+
+node1 = wpan.Node()
+node2 = wpan.Node()
+
+# -----------------------------------------------------------------------------------------------------------------------
+# Init all nodes
+
+wpan.Node.init_all_nodes()
+
+# -----------------------------------------------------------------------------------------------------------------------
+# Build network topology
+
+# Two-node network (node1 leader/router, node2 end-device)
+
+node1.form('horizon')  # "zero dawn"
+node2.join_node(node1, node_type=wpan.JOIN_TYPE_END_DEVICE)
+
+# -----------------------------------------------------------------------------------------------------------------------
+# Test implementation
+
+# Get the link local addresses
+ll1 = node1.get(wpan.WPAN_IP6_LINK_LOCAL_ADDRESS)[1:-1]
+ll2 = node2.get(wpan.WPAN_IP6_LINK_LOCAL_ADDRESS)[1:-1]
+
+PORT = 1234
+MSG_LEN_START = 100
+MSG_LEN_END = 500
+
+for msg_length in range(MSG_LEN_START, MSG_LEN_END):
+    sender = node1.prepare_tx((ll1, PORT), (ll2, PORT), msg_length)
+    recver = node2.prepare_rx(sender)
+
+    wpan.Node.perform_async_tx_rx()
+
+    verify(sender.was_successful)
+    verify(recver.was_successful)
+
+# -----------------------------------------------------------------------------------------------------------------------
+# Test finished
+
+wpan.Node.finalize_all_nodes()
+
+print('\'{}\' passed.'.format(test_name))