Don't fail when we get a UDP checksum of 0xffff.

scapy doesn't appear to change a zero checksum to 0xffff for UDP
packets as required by RFC 768, but unsurprisingly, the kernel
does. That can cause the test to fail because assertPacketMatches
does a byte comparison and sees that 0 != ffff. Fix this by
rewriting ffff to 0 for UDP packets.

Change-Id: I6ab13ae5597dcc8ce34f591076ac7dd87f81a96f