frag6: import a set of test cases

In order to ensure that changing the frag6 code does not change behaviour
or break code a set of test cases were implemented.

Like some other test cases these use Scapy to generate packets and possibly
wait for expected answers. In most cases we do check the global and
per interface (netstat) statistics output using the libxo output and grep
to validate fields and numbers. This is a bit hackish but we currently have
no better way to match a selected number of stats only (we have to ignore
some of the ND6 variables; otherwise we could use the entire list).

Test cases include atomic fragments, single fragments, multi-fragments,
and try to cover most error cases in the code currently.
In addition vnet teardown is tested to not panic.

A separate set (not in-tree currently) of probes were used in order to
make sure that the test cases actually test what they should.

The "sniffer" code was copied and adjusted from the netpfil version
as we sometimes will not get packets or have longer timeouts to deal with.

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