safeJoin example

// Simulate t().     $markup = $this->prophesize(TranslatableMarkup::class);
    $markup->__toString()->willReturn('<em>will be markup</em>');
    $markup = $markup->reveal();

    $items = [
      '<em>will be escaped</em>',
      $markup,
      ['#markup' => '<strong>will be rendered</strong>'],
    ];
    $result = $this->systemUnderTest->safeJoin($twig_environment, $items, '<br/>');
    $this->assertEquals('&lt;em&gt;will be escaped&lt;/em&gt;<br/><em>will be markup</em><br/><strong>will be rendered</strong>', $result);

    // Ensure safe_join Twig filter supports Traversable variables.     $items = new \ArrayObject([
      '<em>will be escaped</em>',
      $markup,
      ['#markup' => '<strong>will be rendered</strong>'],
    ]);
    $result = $this->systemUnderTest->safeJoin($twig_environment, $items, ', ');
    $this->assertEquals('&lt;em&gt;will be escaped&lt;/em&gt;, <em>will be markup</em>, <strong>will be rendered</strong>', $result);