net: buf: Add tests for net_buf fragmentation

More realistic IPv6 packet creation tests created.
One test creates one big IPv6 packet and manipulates that.
Second test creates a list of IPv6 header + data fragments
and sets some user data in those fragment buffers.

Change-Id: Ic8309ff115e1d735e98c30b3c8292b0d6ffc5f42
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>