test/mbuf: check pinned external buffer

This patch adds unit test for the mbufs allocated from
the special pool with pinned external data buffers.

The pinned buffer mbufs are tested in the same way as
regular ones with taking into account some specifics
of cloning/attaching.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>