Correct way to check if a class has a constant defined with PHPUnit

I am trying to find out the best, or correct, way to check if a class has a constant defined with PHPUnit. The PHPUnit docs don't seem to cover this, which makes me wonder if I am doing the correct thing by testing this - however it is an important feature of my class.

I have the following class:

PurchaseManager.php

/**
 * Message sent when a course has been purchased
 */
const COURSE_PURCHASED_MESSAGE = 'coursePurchasedMessage';

...and part of it's test class has this test:

PurchaseManagerTest.php

public function testCoursePurchasedMessageConstant()
{
    $pm = new PurchaseManager();
    $this->assertTrue(defined(get_class($pm) . '::COURSE_PURCHASED_MESSAGE'));
}

Is this correct? It passes, but i'm just interested to know if this is accurate and best practice.

I am using PHPUnit 5.0.8.