public function testContains()
 {
     $item1 = 'item 1';
     $item2 = 'item 2';
     $item3 = 'item 3';
     $items = [$item1, $item2];
     $queue = new Queue($items);
     $this->assertTrue($queue->contains($item2));
     $this->assertFalse($queue->contains($item3));
 }