Phpunit throws an error when dealing with external classes?

Question

PHPUnit

Installed PHPStorm and phpunit.
When you run any primitive test, where there is no access to another file, everything is fine, the test passes:

class firstTest extends PHPUnit_Framework_TestCase
{
    public function testFirst(){
        $this->assertTrue(1 === 1);
    }
}

, but when I also run a simple test, but which tests another class, for example, a calculator

<?php
class Calculator
{

    public function add($a, $b)
    {
        return $a + $b;
    }

}

<?php
require '../Calculator.php';

class CalculatorTests extends PHPUnit_Framework_TestCase
{
    private $calculator;

    protected function setUp()
    {
        $this->calculator = new Calculator();
    }

    protected function tearDown()
    {
        $this->calculator = NULL;
    }

    public function testAdd()
    {
        $result = $this->calculator->add(1, 2);
        $this->assertEquals(3, $result);
    }

}

gives an error Process finished with exit code 255.
As I understand it, for some reason it does not pull up external classes, why?
Help a newbie, thanks in advance.