frag6 tests: set end to catch timeout as error

There are times when we have to wait for reply packets. There are
either an ICMPv6 (error) reply or the expiration timeout.
In these cases synchonous ICMPv6 replies should arrive, always,
unless the packet is lost. Due to errors experienced with the
test software sending an invlaid request on at least i386 (*) these
packets are not generated. That means we are waiting for a long time
for the replies or even timeout the test case.

Manually set the "End" flag on these test cases as well, so they do
fail rather than timeout as the sniffer timeout happens. This improves
debugging options, reflects the error properly, and saves time on each
test suit run.

(*) The real cause for that is still to be found (see the referenced PRs)

PR: 241493, 239380
MFC after: 2 weeks
Sponsored by: Netflix