Allow immediate processing of just prepared tasks in cron worker runner

Preparing simple worker tasks is super fast, because it just switches the state to null.
But the processing of just prepared tasks required the second worker run and also second cron run.
This change optimize the behavior and in case there is still time left it immediately starts processing the tasks.
[MAILPOET-4684]

mailpoet/tests/integration/Cron/CronWorkerRunnerTest.php

@@ -47,10 +47,10 @@ class CronWorkerRunnerTest extends \MailPoetTest {
     $this->cronWorkerRunner->run($worker);
   }
 
-  public function testItPreparesTask() {
+  public function testItPreparesTaskAndProcessesItImmediately() {
     $worker = $this->make(SimpleWorkerMockImplementation::class, [
       'prepareTaskStrategy' => Expected::once(true),
-      'processTaskStrategy' => Expected::never(),
+      'processTaskStrategy' => Expected::once(true),
     ]);
 
     $task = $this->createScheduledTask();
@@ -58,7 +58,7 @@ class CronWorkerRunnerTest extends \MailPoetTest {
     expect($result)->true();
     $scheduledTask = $this->scheduledTasksRepository->findOneById($task->getId());
     assert($scheduledTask instanceof ScheduledTaskEntity);
-    expect($scheduledTask->getStatus())->null();
+    expect($scheduledTask->getStatus())->same(ScheduledTaskEntity::STATUS_COMPLETED);
   }
 
   public function testItProcessesTask() {