GD-152: Implement a TestEngine to discover and execute tests as Godot engine process or inline execution

[MikeSchulze]

Is your feature request related to a problem? Please describe.

Currently, all tests in GdUnitNet are executed within the Godot engine, including unit tests that don't require engine functionality. This leads to unnecessary overhead and slower test execution for pure logic tests that could run independently of the engine.

Describe the solution you'd like

Introduce a way to explicitly mark which tests require the Godot engine runtime, allowing other tests to run more efficiently outside the engine context.

Option A: Extended TestCase Attribute

Add an engine parameter to the existing TestCase attribute:

[TestCase(engine = true)]
public async Task TestWithEngineComponents()
{
    var sceneRunner = ISceneRunner.Load("res://src/core/resources/scenes/TestSceneCSharp.tscn", true);
    await sceneRunner.SimulateFrames(100);
    // Test logic here
}

[TestCase] // or explicitly [TestCase(engine = false)]
public void TestPureLogic()
{
    // Pure C# logic testing
}

Pros:

• No breaking changes to existing tests
• Clear and explicit opt-in for engine-required tests
• Consistent with existing attribute structure

Cons:

• Additional parameter might be overlooked
• Default value needs careful consideration

Option B: New GodotTestCase Attribute

Introduce a new attribute specifically for engine-dependent tests:

[GodotTestCase]
public async Task TestWithEngineComponents()
{
    var sceneRunner = ISceneRunner.Load("res://src/core/resources/scenes/TestSceneCSharp.tscn", true);
    await sceneRunner.SimulateFrames(100);
    // Test logic here
}

[TestCase]
public void TestPureLogic()
{
    // Pure C# logic testing
}

Pros:

• Very clear distinction between test types
• No ambiguity about test requirements
• Follows Single Responsibility Principle

Cons:

• Breaking change requiring migration of existing tests
• Additional attribute to maintain

Error Handling

When a test uses Godot engine features without the appropriate attribute/flag:

Test should fail immediately with a clear error message
Suggested error message: "This test uses Godot engine features but is not marked for engine execution. Please add [GodotTestCase] or [TestCase(engine = true)] to this test."

The framework should detect attempts to use engine features in non-engine tests, including:

• Node operations
• Scene management
• Physics interactions
• Input system usage
• Signal connections
• Resource loading