Fix PlaylistResultsScreen test failures

As seen: https://github.com/ppy/osu/pull/17252/checks?check_run_id=5545717506

The `PlaylistsResultsScreen` takes a lease on the `Beatmap` bindable
when entered. During `SetUp`, the `Beatmap` bindable is reassigned but
fails in cases where an existing test instance of the screen hasn't been
exited yet. This fix moves the assignment into the `SetUpSteps` function
after `base.SetUpSteps` is called which will exit the existing screens
first.

osu.Game.Tests/Visual/Playlists/TestScenePlaylistsResultsScreen.cs

@@ -53,12 +53,23 @@ namespace osu.Game.Tests.Visual.Playlists
             userScore.Statistics = new Dictionary<HitResult, int>();
 
             bindHandler();
-
-            // beatmap is required to be an actual beatmap so the scores can get their scores correctly calculated for standardised scoring.
-            // else the tests that rely on ordering will fall over.
-            Beatmap.Value = CreateWorkingBeatmap(Ruleset.Value);
         });
 
+        [SetUpSteps]
+        public override void SetUpSteps()
+        {
+            base.SetUpSteps();
+
+            // Existing test instances of the results screen hold a leased bindable of the beatmap,
+            // so wait for those screens to be cleaned up by the base SetUpSteps before re-assigning
+            AddStep("Create Working Beatmap", () =>
+            {
+                // Beatmap is required to be an actual beatmap so the scores can get their scores correctly calculated for standardised scoring.
+                // else the tests that rely on ordering will fall over.
+                Beatmap.Value = CreateWorkingBeatmap(Ruleset.Value);
+            });
+        }
+
         [Test]
         public void TestShowWithUserScore()
         {