Convert system functional tests to phpunit /Routing/DestinationTest.php

Follow up from #2982149: Page/Routing/Render/Session: Convert system functional tests to phpunit

First apply patch from #4 then work on this.

At least this needs to be fixed:

$this->drupalGet('system-test/get-destination', ['query' => ['destination' => $test_case['input']]]);
      $this->assertIdentical($test_case['output'], $this->getRawContent(), $test_case['message']);
      // Test $_REQUEST['destination'].
      $post_output = $this->drupalPost('system-test/request-destination', '*', ['destination' => $test_case['input']]);
      $this->assertIdentical($test_case['output'], $post_output, $test_case['message']);