tests: access kernel object with private data using system call

When defining system calls, it is very important to ensure that
access to the API’s private data is done exclusively through system
call interfaces. Private kernel data should never be made available
to user mode threads directly. For example, the k_queue APIs were
intentionally not made available as they store bookkeeping
information about the queue directly in the queue buffers which are
visible from user mode.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>