Fix remaining cases of incorrect GameplayState construction in tests

Manual attempts to initialise replaced by a new `TestGameplayState`
helper for ease of use.

osu.Game/Tests/Gameplay/TestGameplayState.cs

@@ -0,0 +1,32 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+#nullable enable
+
+using System.Collections.Generic;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Scoring;
+using osu.Game.Screens.Play;
+using osu.Game.Tests.Beatmaps;
+
+namespace osu.Game.Tests.Gameplay
+{
+    /// <summary>
+    /// Static class providing a <see cref="Create"/> convenience method to retrieve a correctly-initialised <see cref="GameplayState"/> instance in testing scenarios.
+    /// </summary>
+    public static class TestGameplayState
+    {
+        /// <summary>
+        /// Creates a correctly-initialised <see cref="GameplayState"/> instance for use in testing.
+        /// </summary>
+        public static GameplayState Create(Ruleset ruleset, IReadOnlyList<Mod>? mods = null, Score? score = null)
+        {
+            var beatmap = new TestBeatmap(ruleset.RulesetInfo);
+            var workingBeatmap = new TestWorkingBeatmap(beatmap);
+            var playableBeatmap = workingBeatmap.GetPlayableBeatmap(ruleset.RulesetInfo, mods);
+
+            return new GameplayState(playableBeatmap, ruleset, mods, score);
+        }
+    }
+}