Update SA1206 tests for covariant return types

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp9/OrderingRules/SA1206CSharp9UnitTests.cs

@@ -3,9 +3,62 @@
 
 namespace StyleCop.Analyzers.Test.CSharp9.OrderingRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp8.OrderingRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
+        StyleCop.Analyzers.OrderingRules.SA1206DeclarationKeywordsMustFollowOrder,
+        StyleCop.Analyzers.OrderingRules.SA1206CodeFixProvider>;
 
     public partial class SA1206CSharp9UnitTests : SA1206CSharp8UnitTests
     {
+        [Fact]
+        [WorkItem(3975, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3975")]
+        public async Task TestCovariantOverrideKeywordsOutOfOrderAsync()
+        {
+            var testCode = @"
+public class BaseType
+{
+}
+
+public class DerivedType : BaseType
+{
+}
+
+public class BaseClass
+{
+    public virtual BaseType Create() => new BaseType();
+}
+
+public class DerivedClass : BaseClass
+{
+    override [|public|] DerivedType Create() => new DerivedType();
+}
+";
+
+            var fixedCode = @"
+public class BaseType
+{
+}
+
+public class DerivedType : BaseType
+{
+}
+
+public class BaseClass
+{
+    public virtual BaseType Create() => new BaseType();
+}
+
+public class DerivedClass : BaseClass
+{
+    public override DerivedType Create() => new DerivedType();
+}
+";
+
+            await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }