Add tests for SDL_error.cs

SDL3-CS.Tests/TestError.cs

@@ -0,0 +1,25 @@
+// 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;
+
+namespace SDL.Tests
+{
+    [TestFixture]
+    public class TestError
+    {
+        [Test]
+        public void TestUnsupported()
+        {
+            Assert.That((bool)SDL3.SDL_Unsupported(), Is.False);
+            Assert.That(SDL3.SDL_GetError(), Is.EqualTo("That operation is not supported"));
+        }
+
+        [Test]
+        public void TestInvalidParam()
+        {
+            Assert.That((bool)SDL3.SDL_InvalidParamError("test"), Is.False);
+            Assert.That(SDL3.SDL_GetError(), Is.EqualTo("Parameter 'test' is invalid"));
+        }
+    }
+}