Make SchemaCheckTestTrait available outside of the config module

@dawehner,

I've been looking into this issue. But we're using this Trait in about 73 different places. What do you exactly mean with "let's keep the old one as an alias"? Should we copy the existing trait to the new location and keep the old file in the place where it now is? This will introduce code duplication, right?

<?php
namespace Drupal\Core\Config\Schema {
    trait OldTrait {
        public function thing() {
        }
    }
}

namespace Drupal\Tests {
    trait NewTrait {
        use \Drupal\Core\Config\Schema\OldTrait;
    }
}

namespace {
    class Foo {
        use \Drupal\Tests\NewTrait;
    }
}

Thanks for the help #5!

Used the example of #5 a bit too literally.

What? How & why?

+++ b/core/modules/config/src/Tests/SchemaCheckTestTrait.php
@@ -2,58 +2,11 @@
 /**
  * Provides a class for checking configuration schema.
  */

The original trait needs a deprecation notice and we should update core usages here too.

I have added the deprecated message and updated all core usages to use the new namespace.

Applied this match and ran all PHPUnit tests locally (which technically is just doing my human hands what testbot already did), and can confirm it works.

Committed/pushed to 8.3.x, thanks!