Add tests for Views fields from relationship

I'm glad that the tests agree with these changes and I like the additional coverage. It would be uncharacteristic to not find anything that could use slight improvements. Overall I think this patch is in great shape though, since I don't see any actual code that can be improved. I'm personally not a fan of the call_user_func_array call in the code, as that part doesn't read very well. However this is just moved around so not really something we should discuss/resolve in this particular issue.

1. +++ b/src/Plugin/views/field/SearchApiEntity.php
   @@ -280,4 +283,21 @@ protected function getItem(EntityInterface $entity) {
   +   * @return string|null
   

   I think this should be string|NULL, however I see both used in core everywhere.

2. +++ b/src/Plugin/views/field/SearchApiEntityField.php
   @@ -98,14 +98,46 @@ protected function getParentPath() {
   +    // The probably easiest way to do this is just use the $options we got from
   +    // the parent, but exclude those that come directly from the parent.
   

   This feels like a weird statement "The probably easiest way..."

   Can we replace that with: "We use the $options array from the parent, but exclude the items that come directly from the parent." or something similar?

3. +++ b/src/Plugin/views/field/SearchApiFieldTrait.php
   @@ -390,23 +390,22 @@ public function preRender(&$values) {
   +            // by merging them together (currently it's an array of arrays, but
   +            // it should be just a flat array).
   

   Since we're touching this code anyway, let's add a @todo with a link to a d.o issue or remove the todo-part of the comment.

4. +++ b/src/Plugin/views/field/SearchApiFieldTrait.php
   @@ -626,4 +625,14 @@ protected function getItemUrl(ResultRow $row, $i) {
   +   * Returns the Render API renderer.
   

   I can see this is something that's also described as "the render API renderer" in other places but this is the first time I've seen that. We can keep this though.

5. +++ b/src/Tests/ViewsTest.php
   @@ -190,16 +190,31 @@ protected function checkResults(array $query, array $expected_results = NULL, $l
   +    // For viewing the user name and roles of the user associated with test
   +    // entities, the logged-in user needs to have the permission to administer
   +    // both users and permissions.
   

   That's a really awesome comment! @drunken monkey++

6. +++ b/src/Tests/ViewsTest.php
   @@ -190,16 +190,31 @@ protected function checkResults(array $query, array $expected_results = NULL, $l
   +    // Set the user IDs associated with our test entities.
   

   I think this can benefit from a better description, how about: "To test the entity permissions, attach users with different permission sets to the entities"

7. +++ b/src/Tests/ViewsTest.php
   @@ -253,7 +270,24 @@ public function testViewsAdmin() {
   +    // @todo For some strange reason, the "roles" field form is not included
   +    //   automatically in the series of field forms shown to us by Views. Deal
   +    //   with this graciously (since it's not really our fault, I hope), but it
   +    //   would be great to have this working normally.
   

   Should we resolve this in this issue? We probably should at least figure out if we're doing something wrong and adjust the comment accordingly. At the very least we can open a new issue for that.

8. +++ b/src/Tests/ViewsTest.php
   @@ -301,10 +344,17 @@ public function testViewsAdmin() {
   +   * @return string|null
   

   See my other comment about this.