Add tests for source generators

- Bootstrap xUnit project to test source generators
- Implement source generator tests
- Better tests structure (put test data in cs files)
- Enable `ScriptSerializationGeneratorTests`
- Enable `ScriptPathAttributeGeneratorTests`
- Fix `NesterClass` -> `NestedClass`
- Use `Path.Combine` when dealing with paths
- Copy test data to the output directory

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/Extensions.cs

@@ -0,0 +1,12 @@
+using System.Reflection;
+using Microsoft.CodeAnalysis;
+
+namespace Godot.SourceGenerators.Tests;
+
+public static class Extensions
+{
+    public static MetadataReference CreateMetadataReference(this Assembly assembly)
+    {
+        return MetadataReference.CreateFromFile(assembly.Location);
+    }
+}