Follow-up for #2953321; fixes PHP 5.5 test failure

#2953321: Comprehensive JSON API integration test coverage phase 5: nested includes and sparse field sets caused HEAD to break on PHP 5.5.

That's because of the string type hint below:

+++ b/tests/src/Functional/ResourceResponseTestTrait.php
@@ -121,6 +130,55 @@ trait ResourceResponseTestTrait {
+    $resource_data = array_reduce($include_paths, function ($data, string $path) use ($request_options) {

This is a test-only issue.