Change folder structure of test

I just ran into this with the gdextension work we're doing but it applies to master as well.

Currently the test project embedded here has the godot project file in the root and the source for the plugin in a `src` folder.
On Windows this causes the issue that `init.cpp` gets compiled to `init.obj`
Because this is inside of the Godot project folder, the Godot editor sees this as a wavefront obj file and attempts to import it as a 3D file.

I suggest changing the folder structure as follows:
```
- test
- demo
- gdexample.gdnlib
- gdexample.gdns
- project.godot
- script.gd
- src
- init.cpp
- SConstruct
```

This also means the binaries should build into into `test/demo/bin/`