Replace instance_of asserts in integration tests

[MAILPOET-4258]

mailpoet/tests/integration/Segments/SegmentSubscribersRepositoryTest.php

@@ -70,9 +70,9 @@ class SegmentSubscribersRepositoryTest extends \MailPoetTest {
     $wpUserSubscriber = $this->entityManager
       ->getRepository(SubscriberEntity::class)
       ->findOneBy(['email' => $wpUserEmail]);
-    assert($wpUserSubscriber instanceof SubscriberEntity);
+    $this->assertInstanceOf(SubscriberEntity::class, $wpUserSubscriber);
     $wpUserSubscriber->setStatus(SubscriberEntity::STATUS_SUBSCRIBED);
-    assert($wpUserSubscriber instanceof SubscriberEntity);
+    $this->assertInstanceOf(SubscriberEntity::class, $wpUserSubscriber);
     $subscriberNoList = $this->createSubscriberEntity(); // Subscriber without segment
     $this->entityManager->flush();