Convert tests/arp-too-long-tha.pcap to a pcap file.

Convert tests/arp-too-long-tha.pcap to a pcap file.

It was, its name nonwithstanding, a pcapng file, and had an SHB option
that was damaged by fuzzing.  Converting it to pcap

1) means it has the appropriate extension;

2) means that old libpcaps that can't read pcapng can handle it;

3) means that a future libpcap that fully supports pcapng *and* thus
fails on that file can handle it.

(cherry picked from commit 9075a2857ef6e69e2d8d948a72d51cb68a39c026)