CodeIgniter with PHPUnit

I'm trying to apply PHPUnit in my project with CodeIgniter, based on the following example ( link ), until then when the installation and dependency files are all ok, only when I add setUp () method.

<?php
class ApiTest extends PHPUnit_Framework_TestCase {
    private $CI;

    public function setUp() {
        $this->CI = &get_instance();
    }

    public function testVerifyFormat() {
        $data = array(
            "token" => "895c0d9b87d5ba256268f7238b61771e2b10291085ecb1015681817cb428fb66",
            "updated_registries" => 30
        );
        $expected = json_encode($data, true);

        $this->assertEquals(array_keys($data)[0], "token");
        $this->assertEquals(array_keys($data)[1], "updated_registries");
        $this->assertEquals(2, count($data));
    }
}

It displays the following error:

Fatal error: "Call to undefined function get_instance()" in C:\xampp\htdocs\devops_ci\tests\ApiTest.php on line 7

Folder structure

• application
• assets
• system
• test (Bootstrap.php ApiTest.php)
• user_guide
• vendor (php folder downloaded after running composer.json)
• phpunit.xml
• index.php

I can run the tests, and I'm not able to reference the controllers that are within application , and by what I understood this method setUp () does just that.