frag6-test: update for r354046, conform to 8200 for overlapping fragments

The change to conform to RFC 8200 for overlapping fragments now frees
the entire reassembly queue if the overlapping fragments are not an
exact match.
As a result we do see one less packet in the timeout statistics from
expiry. No other statistics change as the event is not counted.
It can be argued that we should improve the statistics counters in
that case.
This test case update should have been committed alongside the original
commit.

Pointyhat to: bz
MFC after: 3 weeks
X-MFC with: r354046
Sponsored by: Netflix