Replace instance_of asserts in integration tests

[MAILPOET-4258]

mailpoet/tests/integration/API/JSON/ResponseBuilders/DynamicSegmentsResponseBuilderTest.php

@@ -83,7 +83,7 @@ class DynamicSegmentsResponseBuilderTest extends \MailPoetTest {
     $wpUserSubscriber = $this->entityManager
       ->getRepository(SubscriberEntity::class)
       ->findOneBy(['email' => $wpUserEmail]);
-    assert($wpUserSubscriber instanceof SubscriberEntity);
+    $this->assertInstanceOf(SubscriberEntity::class, $wpUserSubscriber);
     $wpUserSubscriber->setStatus(SubscriberEntity::STATUS_SUBSCRIBED);
     $segment = $this->createDynamicSegmentEntity($name, $description);
     $this->addDynamicFilter($segment, 'editor');