Kernel test coverage for assignment plugins

In general the tests are looking great! Small tests, and limits in its scope.

1. +++ b/tests/src/Kernel/FeaturesAssignTest.php
   @@ -0,0 +1,225 @@
   +  /**
   +   * @var \Drupal\features\FeaturesManagerInterface
   +   */
   +  protected $featuresManager;
   ...
   +  /**
   +   * @var \Drupal\features\FeaturesAssignerInterface
   +   */
   +  protected $assigner;
   ...
   +    $this->assigner = \Drupal::service('features_assigner');
   

   Given that features knows which code exists I'd have typehinted against concrete classes, which makes it a bit easier to follow.

2. +++ b/tests/src/Kernel/FeaturesAssignTest.php
   @@ -0,0 +1,225 @@
   +    $this->addConfigurationItem('node.type.article', [], [
   

   yeah, for adding helper methods here.

3. +++ b/tests/src/Kernel/FeaturesAssignTest.php
   @@ -0,0 +1,225 @@
   +    $this->assertEquals($expected_package_names, array_keys($packages), 'Expected packages not created.');  ¶
   ...
   +    $this->assertEquals($expected_config_items, $packages['article']['config'], 'Expected configuration items not present in article package.');  ¶
   

   There are a little bit of whitespace ... I'd also just not use messages anymore, its kinda pointless IMHO quite often.

4. +++ b/tests/src/Kernel/FeaturesAssignTest.php
   @@ -0,0 +1,225 @@
   +    $this->assertTrue(in_array('field.storage.node.body', $packages['core']['config'], 'Expected configuration item not present in core package.'));  ¶
   +    $this->assertFalse(in_array('field.field.node.article.body', $packages['core']['config'], 'Unexpected configuration item present in core package.'));  ¶
   

   There is assertContains if you want to use that. Its a bit better to read at the end.

Fixed those nitpicks and committed to 8.x-3.x

Thank you!