Error executing PHPUnit
I was able to resolve the issue. You just have to prefix your test with test_. Seriously! No kidding!

My test looks like the following:

&lt;?php
/**
 * @file
 * Contains Drupal\Tests\phpunit_d8\Unit\MathTest
 */

namespace Drupal\Tests\phpunit_d8\Unit;

use Drupal\Tests\UnitTestCase;
use Drupal\phpunit_d8\Math;

/**
 * Class MathTest
 * @package Drupal\Tests\phpunit_d8\Unit
 *
 * @group math
 */
class MathTest extends UnitTestCase {
  /**
   * Very simple test to MathTest::addNum().
   */
  public function testAddNum() {
    $math = new Math();
    $this-&gt;assertEquals($math-&gt;addNum(1,2,3), 6);
  }
}


Also, pushed at https://github.com/ajitdev/phpunit_d8