Add failing tests for SDL_mixer

They don't compile because of missing friendly overloads.

SDL3-CS.Tests/TestMixer.cs

@@ -0,0 +1,41 @@
+// 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 NUnit.Framework;
+using static SDL.SDL3_mixer;
+using static SDL.SDL3;
+
+namespace SDL.Tests
+{
+    [TestFixture]
+    public class TestMixer
+    {
+        [Test]
+        public unsafe void TestBasic()
+        {
+            SDL_Init(0);
+
+            bool init = MIX_Init();
+
+            try
+            {
+                Assert.That(init, Is.True, SDL_GetError);
+                Assert.That(MIX_Version(), Is.EqualTo(SDL_MIXER_VERSION));
+
+                Assume.That(MIX_GetNumAudioDecoders() > 0);
+                string? name = MIX_GetAudioDecoder(0);
+                Assert.That(name, Is.Not.Null, SDL_GetError);
+
+                Assume.That(@"C:\Windows\Media\Windows Logon.wav", Does.Exist);
+                var decoder = MIX_CreateAudioDecoder(@"C:\Windows\Media\Windows Logon.wav", 0);
+                Assert.That(decoder != null, SDL_GetError);
+                MIX_DestroyAudioDecoder(decoder);
+            }
+            finally
+            {
+                MIX_Quit();
+                SDL_Quit();
+            }
+        }
+    }
+}