selftest: net: Improve IPV6_TCLASS/IPV6_HOPLIMIT tests apparmor compatibility

"tcpdump" is used to capture traffic in these tests while using a random,
temporary and not suffixed file for it. This can interfere with apparmor
configuration where the tool is only allowed to read from files with
'known' extensions.

The MINE type application/vnd.tcpdump.pcap was registered with IANA for
pcap files and .pcap is the extension that is both most common but also
aligned with standard apparmor configurations. See TCPDUMP(8) for more
details.

This improves compatibility with standard apparmor configurations by
using ".pcap" as the file extension for the tests' temporary files.

Signed-off-by: Andrei Gherzan <andrei.gherzan@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>