svm: fix multi-chunk fifo alloc and add more tests

svm: fix multi-chunk fifo alloc and add more tests

Type: fix

- make sure that chunks and the rbtree are initialized if fifo segment
allocates multiple chunks for the fifo.
- ensure head/tail chunks are updated on all enqueue/dequeue events,
including when dropping data.
- more unit tests

Also fixes dequeue drop updates of head chunk.

Change-Id: I77f3550bc4e8b4e077f80ea87fe82b83ed013aeb
Signed-off-by: Florin Coras <fcoras@cisco.com>