Convert \Drupal\Tests\system\Functional\Common\FormatDateTest mostly to a kernel test

Came across this while reviewing datetime issues. Here's a quick conversion to kernel. I left the other test method in place since it covers the UI. Do we want to remove the calls to format_date and call the service directly as part of this?

The split of WTB vs KTB is good. I think this is essentially good to go, just some small nits since we have a new test class, we can update things.

1. +++ b/core/tests/Drupal/KernelTests/Core/Datetime/FormatDateTest.php
   @@ -0,0 +1,90 @@
   +      '' => array('Sunday' => 'domingo'),
   +      'Long month name' => array('March' => 'marzo'),
   

   Short array syntax.

2. +++ b/core/tests/Drupal/KernelTests/Core/Datetime/FormatDateTest.php
   @@ -0,0 +1,90 @@
   +  function testFormatDate() {
   

   PHPCS complained that this should be explicitly public.

3. +++ b/core/tests/Drupal/KernelTests/Core/Datetime/FormatDateTest.php
   @@ -0,0 +1,90 @@
   +    $this->assertSame(format_date($timestamp, 'custom', 'l, d-M-y H:i:s T', 'America/Los_Angeles', 'en'), 'Sunday, 25-Mar-07 17:00:00 PDT', 'Test all parameters.');
   

   Since this is a new file, I think it is a good idea to go ahead and replace the `format_date` calls with the $formatter from the container. Fewer places to update once we finally start to remove this (this will get rid of about 20% of the current usages).

This should address #3. While I was at it, I also changed the order of the assertion arguments to be correct (expected, then actual).

Looks good to me. I think this would qualify for 8.2 under https://www.drupal.org/core/d8-allowed-changes#patch per

Internal code cleanup that improves maintainability and is not disruptive may sometimes be backported to a patch release, but this is always at committer discretion.

but no big deal if others disagree.

Committed and pushed 5c4a8df to 8.3.x and c30416f to 8.2.x. Thanks!