Add failing test for proxying derived record type

On .NET 6+ this fails with a `TypeLoadException`:

> Return type in method `DerivedEmptyRecord.<Clone>$()` [...] is not
> compatible with base type method `EmptyRecord.<Clone>$()`

From dotnet/efcore#26602 (comment):

> The C# compiler changed the emit strategy for records in .NET 6 to
> take advantage of covariant returns. The Clone method now always has a
> return type that matches the containing type. Covariant returns were
> added to the runtime and language in .NET 5 but records didn't take
> advantage of them (just ran out of time). In .NET 6 though we finished
> off that feature and added it to records.

So we'll need to add support for covariant returns to DynamicProxy.

Author: stakx

src/Castle.Core.Tests/DynamicProxy.Tests/Records/DerivedEmptyRecord.cs

@@ -0,0 +1,20 @@
+// Copyright 2004-2022 Castle Project - http://www.castleproject.org/
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+namespace Castle.DynamicProxy.Tests.Records
+{
+	public record DerivedEmptyRecord : EmptyRecord
+	{
+	}
+}

src/Castle.Core.Tests/DynamicProxy.Tests/RecordsTestCase.cs

@@ -26,5 +26,11 @@ public void Can_proxy_empty_record()
 		{
 			_ = generator.CreateClassProxy<EmptyRecord>(new StandardInterceptor());
 		}
+
+		[Test]
+		public void Can_proxy_derived_empty_record()
+		{
+			_ = generator.CreateClassProxy<DerivedEmptyRecord>(new StandardInterceptor());
+		}
 	}
 }