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/ScriptSerializationGeneratorTests.cs

@@ -0,0 +1,15 @@
+using Xunit;
+
+namespace Godot.SourceGenerators.Tests;
+
+public class ScriptSerializationGeneratorTests
+{
+    [Fact]
+    public async void ScriptBoilerplate()
+    {
+        await CSharpSourceGeneratorVerifier<ScriptSerializationGenerator>.VerifyNoCompilerDiagnostics(
+            "ScriptBoilerplate.cs",
+            "ScriptBoilerplate_ScriptSerialization.generated.cs", "OuterClass.NestedClass_ScriptSerialization.generated.cs"
+        );
+    }
+}