Move \Drupal\Tests\system\Functional\Datetime\DrupalDateTimeTest::testTimezoneFormat into a Kernel test

Drupal\Tests\system\Functional\Datetime\DrupalDateTimeTest currently says this in setup:

  /**
   * Test setup.
   */
  protected function setUp(): void {
    parent::setUp();

  }

So that's a bug in that it's an unnecessary function, which is different from the scope here. But it certainly solves this issue. :-)

There's also a unit test for Drupal\Core\Datetime\DrupalDateTime which fills me with a warm glow.

Crediting @mstrelan and @Mile23 for their time triaging this in #bugsmash

larowlan → credited mstrelan → .