[Impeller] added missing openplayground (flutter#47338)

Not sure how this ended up left off.

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impeller/aiks/aiks_unittests.cc

@@ -3757,6 +3757,7 @@ TEST_P(AiksTest, BlurHasNoEdge) {
           },
   };
   canvas.DrawRect(Rect{300, 300, 200, 200}, blur);
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
 TEST_P(AiksTest, EmptySaveLayerIgnoresPaint) {