Test field extraction in processors

Only nitpicks, overall this look good, I'll have another look tomorrow, when I'm not as tired.

1. +++ b/src/Processor/ProcessorPluginBase.php
   @@ -204,6 +204,112 @@ protected function filterForPropertyPath(array $fields, $property_path) {
   +   * @param \Drupal\search_api\Item\ItemInterface[] $items
   +   *   The items from which properties should be extracted.
   

   An array of items, so to be clear in the description that this is an actual array?

2. +++ b/src/Processor/ProcessorPluginBase.php
   @@ -204,6 +204,112 @@ protected function filterForPropertyPath(array $fields, $property_path) {
   +    $ret = array();
   

   Can we rename this? At least $return, but preferrably to $extracted_values?

3. +++ b/src/Processor/ProcessorPluginBase.php
   @@ -204,6 +204,112 @@ protected function filterForPropertyPath(array $fields, $property_path) {
   +        }
   +        foreach ($required_properties[$datasource_id] as $property_path => $combined_id) {
   

   can we add a newline here? for reability?

4. +++ b/src/Processor/ProcessorPluginBase.php
   @@ -204,6 +204,112 @@ protected function filterForPropertyPath(array $fields, $property_path) {
   +          // don't want to load anything.
   

   Field/property value is not present on the item. Skip this item if we don't want to load it.