AC-15048::[CE] PHPUnit 12: Fix EsConfigTest mock data structure

Author: cod43156

app/code/Magento/Elasticsearch/Test/Unit/Model/Adapter/Index/Config/EsConfigTest.php

@@ -64,7 +64,10 @@ protected function setUp(): void
         $this->serializerMock->expects($this->once())
             ->method('unserialize')
             ->with('serializedData')
-            ->willReturn(['unserializedData']);
+            ->willReturn([
+                'stemmerInfo' => [],
+                'stopwordsInfo' => []
+            ]);
 
         $this->config = $this->objectManager->getObject(
             EsConfig::class,