Add Fragmented Packet Test to testTetherClatBpfOffloadUdp

Extend testTetherClatBpfOffloadUdp to support fragmented IPv6 packets.
This verifies that downstream fragmented IPv6 UDP packets are
correctly processed by Clatd's BPF offload.

Bug: 285124667
Test: atest TetheringIntegrationTests
Change-Id: Id9c6cfac6fcedbc3615a5502e36851bff62e4dcc
3 files changed