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/CSharpSourceGeneratorVerifier.cs

@@ -0,0 +1,82 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Testing;
+using Microsoft.CodeAnalysis.Testing;
+using Microsoft.CodeAnalysis.Testing.Verifiers;
+using Microsoft.CodeAnalysis.Text;
+
+namespace Godot.SourceGenerators.Tests;
+
+public static class CSharpSourceGeneratorVerifier<TSourceGenerator>
+where TSourceGenerator : ISourceGenerator, new()
+{
+    public class Test : CSharpSourceGeneratorTest<TSourceGenerator, XUnitVerifier>
+    {
+        public Test()
+        {
+            ReferenceAssemblies = ReferenceAssemblies.Net.Net60;
+
+            SolutionTransforms.Add((Solution solution, ProjectId projectId) =>
+            {
+                Project project = solution.GetProject(projectId)!
+                    .AddMetadataReference(Constants.GodotSharpAssembly.CreateMetadataReference());
+
+                return project.Solution;
+            });
+        }
+    }
+
+    public static Task Verify(string source, params string[] generatedSources)
+    {
+        return Verify(new string[] { source }, generatedSources);
+    }
+
+    public static Task VerifyNoCompilerDiagnostics(string source, params string[] generatedSources)
+    {
+        return VerifyNoCompilerDiagnostics(new string[] { source }, generatedSources);
+    }
+
+    public static Task Verify(ICollection<string> sources, params string[] generatedSources)
+    {
+        return MakeVerifier(sources, generatedSources).RunAsync();
+    }
+
+    public static Task VerifyNoCompilerDiagnostics(ICollection<string> sources, params string[] generatedSources)
+    {
+        var verifier = MakeVerifier(sources, generatedSources);
+        verifier.CompilerDiagnostics = CompilerDiagnostics.None;
+        return verifier.RunAsync();
+    }
+
+    public static Test MakeVerifier(ICollection<string> sources, ICollection<string> generatedSources)
+    {
+        var verifier = new Test();
+
+        verifier.TestState.AnalyzerConfigFiles.Add(("/.globalconfig", $"""
+        is_global = true
+        build_property.GodotProjectDir = {Constants.ExecutingAssemblyPath}
+        """));
+
+        verifier.TestState.Sources.AddRange(sources.Select(source =>
+        {
+            return (source, SourceText.From(File.ReadAllText(Path.Combine(Constants.SourceFolderPath, source))));
+        }));
+
+        verifier.TestState.GeneratedSources.AddRange(generatedSources.Select(generatedSource =>
+        {
+            return (FullGeneratedSourceName(generatedSource), SourceText.From(File.ReadAllText(Path.Combine(Constants.GeneratedSourceFolderPath, generatedSource)), Encoding.UTF8));
+        }));
+
+        return verifier;
+    }
+
+    private static string FullGeneratedSourceName(string name)
+    {
+        var generatorType = typeof(TSourceGenerator);
+        return Path.Combine(generatorType.Namespace!, generatorType.FullName!, name);
+    }
+}