Add osu! difficulty calculator test

osu.Game/Tests/Beatmaps/DifficultyCalculatorTest.cs

@@ -0,0 +1,44 @@
+// 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.
+
+using System;
+using System.IO;
+using System.Reflection;
+using NUnit.Framework;
+using osu.Game.Beatmaps;
+using osu.Game.Beatmaps.Formats;
+using osu.Game.Rulesets.Difficulty;
+using osu.Game.Rulesets.Mods;
+
+namespace osu.Game.Tests.Beatmaps
+{
+    [TestFixture]
+    public abstract class DifficultyCalculatorTest
+    {
+        private const string resource_namespace = "Testing.Beatmaps";
+
+        protected abstract string ResourceAssembly { get; }
+
+        protected void Test(double expected, string name, params Mod[] mods)
+            => Assert.AreEqual(expected, CreateDifficultyCalculator(getBeatmap(name)).Calculate(mods).StarRating);
+
+        private WorkingBeatmap getBeatmap(string name)
+        {
+            using (var resStream = openResource($"{resource_namespace}.{name}.osu"))
+            using (var stream = new StreamReader(resStream))
+            {
+                var decoder = Decoder.GetDecoder<Beatmap>(stream);
+                ((LegacyBeatmapDecoder)decoder).ApplyOffsets = false;
+                return new TestWorkingBeatmap(decoder.Decode(stream));
+            }
+        }
+
+        private Stream openResource(string name)
+        {
+            var localPath = Path.GetDirectoryName(Uri.UnescapeDataString(new UriBuilder(Assembly.GetExecutingAssembly().CodeBase).Path));
+            return Assembly.LoadFrom(Path.Combine(localPath, $"{ResourceAssembly}.dll")).GetManifestResourceStream($@"{ResourceAssembly}.Resources.{name}");
+        }
+
+        protected abstract DifficultyCalculator CreateDifficultyCalculator(WorkingBeatmap beatmap);
+    }
+}