Improve IPv4PacketBuffer multi-packets tests
Improve IPv4PacketBuffer multi-packets tests

Use different packet length and headers so that parse_multi_packets()
detects issues with ptr::copy().

Genymobile/gnirehtet,WPO-Foundation/gnirehtet,Genymobile/gnirehtet,Genymobile/gnirehtet,WPO-Foundation/gnirehtet,WPO-Foundation/gnirehtet,WPO-Foundation/gnirehtet