net: tests: Add unit tests for net_nbuf_compact()

The compact function was never really properly tested
but it is used by other routines that have unit tests.
This commit makes sure there is a special testing for
the buffer compact function.

Change-Id: I35c108fd8459a9482ccae68a51ffa28fb2e0ae99
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>