tests: sets: Check rbtree overlap detection after tree rotations

tests: sets: Check rbtree overlap detection after tree rotations

Ticket https://bugzilla.netfilter.org/show_bug.cgi?id=1449 showed
an issue with rbtree overlap detection coming from the fact that,
after tree rotations performed as part of tree rebalancing, caused
by deletions, end elements are not necessarily descendants of their
corresponding start elements.

Add single-sized elements, delete every second one of them, and
re-add them (they will always be full overlaps) in order to check
overlap detection after tree rotations.

Port indices used in the sets are pseudo-random numbers generated
with Marsaglia's Xorshift algorithm with triplet (5, 3, 1), chosen
for k-distribution over 16-bit periods, which gives a good
statistical randomness and forces 201 rebalancing operations out of
250 deletions with the chosen seed (1).

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>