Fix flaky segments test

This is hard to debug, but the id was NULL in the previous implementation
and that sometimes returned an object instead of expected
Now the id is always a number and that will hopefully fix the flakyness.

[MAILPOET-1578]

tests/integration/API/JSON/v1/SegmentsTest.php

@@ -124,7 +124,7 @@ class SegmentsTest extends \MailPoetTest {
   }
 
   function testItCanBulkDeleteSegments() {
-    $subscriber_segment = SubscriberSegment::create(array(
+    $subscriber_segment = SubscriberSegment::createOrUpdate(array(
       'subscriber_id' => 1,
       'segment_id' => $this->segment_1->id,
       'status' => Subscriber::STATUS_SUBSCRIBED