Relax caught object stacking test

The stacking code currently uses an unseeded RNG and there is a non-zero chance the stack will be very flat (small Y position difference).
Technically, `RNG.NextSingle(0, 5)` can return `0`, but extremely unlikely that the all RNG calls return 0.

osu.Game.Rulesets.Catch.Tests/TestSceneCatcher.cs

@@ -196,7 +196,7 @@ public void TestCatcherStackingSameCaughtPosition()
             AddAssert("caught objects are stacked", () =>
                 catcher.CaughtObjects.All(obj => obj.Y <= 0) &&
                 catcher.CaughtObjects.Any(obj => obj.Y == 0) &&
-                catcher.CaughtObjects.Any(obj => obj.Y < -25));
+                catcher.CaughtObjects.Any(obj => obj.Y < 0));
         }
 
         [Test]