Make TestUPGrad extend StrongStationarityProperty

Author: ValerianRey

tests/unit/aggregation/test_upgrad.py

@@ -8,6 +8,7 @@
     LinearUnderScalingProperty,
     NonConflictingProperty,
     PermutationInvarianceProperty,
+    StrongStationarityProperty,
 )
 
 
@@ -17,6 +18,7 @@ class TestUPGrad(
     NonConflictingProperty,
     PermutationInvarianceProperty,
     LinearUnderScalingProperty,
+    StrongStationarityProperty,
 ):
     pass