Convert \Drupal\system\Tests\System\CronQueueTest to a kerneltest

1. +++ b/core/modules/system/tests/src/Kernel/System/CronQueueTest.php
   @@ -30,20 +58,23 @@ public function testExceptions() {
   +    // Expire the queue item manually. system_cron() relies in REQUEST_TIME to
   +    // find queue items whose expire field needs to be reset to 0. This is a
   +    // Kernel test, so REQUEST_TIME won't change when cron runs.
   +    // @see system_cron()
        // @see \Drupal\Core\Cron::processQueues()
   

   We could clean that up in https://www.drupal.org/node/2717207 a bit, I think, but for now this seems okay/the only solution to test that?

2. +++ b/core/modules/system/tests/src/Kernel/System/CronQueueTest.php
   @@ -57,7 +88,8 @@ public function testExceptions() {
        // item.
   -    $this->cronRun();
   +    system_cron();
   +    $this->cron->run();
   
   @@ -72,7 +104,9 @@ public function testExceptions() {
        $queue->createItem([]);
   -    $this->cronRun();
   +    system_cron();
   +    $this->cron->run();
   

   Why does $this->cron->run() not execute system_cron()?
   Do we care that this bit is actually implemented in system module itself?

Thank you!