Prepare processor tests to be executed on different backends

There were 2 failures:

1) Drupal\Tests\search_api\Kernel\Processor\AddHierarchyTest::testPreprocessIndexItemsTaxonomy
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
 Array (
-    0 => 'entity:node/1:en'
-    1 => 'entity:node/2:en'
+    0 => 'entity:node/2:en'
 )

/var/www/html/core/tests/Drupal/KernelTests/KernelTestBase.php:1168
/var/www/html/modules/search_api/tests/src/Kernel/ResultsTrait.php:43
/var/www/html/modules/search_api/tests/src/Kernel/Processor/AddHierarchyTest.php:210

2) Drupal\Tests\search_api\Kernel\Processor\AddHierarchyTest::testPreprocessIndexItems
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
 Array (
     0 => 'entity:node/5:en'
     1 => 'entity:node/6:en'
-    2 => 'entity:node/7:en'
-    3 => 'entity:node/8:en'
-    4 => 'entity:node/9:en'
 )

/var/www/html/core/tests/Drupal/KernelTests/KernelTestBase.php:1168
/var/www/html/modules/search_api/tests/src/Kernel/ResultsTrait.php:43
/var/www/html/modules/search_api/tests/src/Kernel/Processor/AddHierarchyTest.php:299

I get exactly the same errors on the Solr backend!