An easy way to do this is to use CakePHP build to add your package folder, then just refer to it by the name you chose so for example:
Add this to bootstrap.php
:
/**
* Add New Package Locations
*/
App::build(['Controller/Trait' => [APP . 'Controller' . DS . 'Trait' . DS]], App::REGISTER);
Then just use this to pull in your traits:
App::uses('TestTrait', 'Controller/Trait');
You can also add multiple package folders at a time so you can apparently do something like this:
/**
* Add New Package Locations
*/
App::build(['All/Trait' => [
APP . 'Controller' . DS . 'Trait' . DS,
APP . 'Model' . DS . 'Trait' . DS,
APP . 'Lib' . DS . 'Trait' . DS
]], App::REGISTER);