Added validation test for the KTX2 decoder

tests/validation/config.json

@@ -890,6 +890,11 @@
             "playgroundId": "#SYQW69#311",
             "referenceImage": "objTestLoader.png",
             "renderCount": 10
+        },
+        {
+            "title": "KTX2 decoder test",
+            "playgroundId": "#EIJH8L#1",
+            "referenceImage": "ktx2decoder.png"
         }
     ]
 }