kunit: Add example for using test->priv

In a test->priv field the user can store arbitrary data.
Add example how to use this feature in the test code.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: David Gow <davidgow@google.com>
Cc: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>