Fix typo in EntityReferenceFieldTest::testReferencedEntitiesMultipleLoad()

Problem/Motivation

$loaded_entities doesn't exist in EntityReferenceFieldTest::testReferencedEntitiesMultipleLoad()

diff --git a/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php b/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php
index 14a67ef..4f66b4e 100644
--- a/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php
+++ b/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php
@@ -170,7 +170,7 @@ public function testReferencedEntitiesMultipleLoad() {
       else {
         // A non-existent or NULL entity target id must not return any item in
         // the target entities set.
-        $this->assertFalse(isset($loaded_entities[$delta]));
+        $this->assertFalse(isset($entities[$delta]));
       }
     }
   }

Proposed resolution

Fix it.

Remaining tasks

Patch it commit it.

User interface changes

None

API changes

None

Data model changes

None.