Set up automated testing and test Copy Files task

[svetogam]

This is an example test for #10.

It runs the Execution scene on a project that has previously been set up in Project Builder. The test projects are kept in a folder with a .gdignore file so they don't mess up opening Project Builder in Godot.

This general testing template can be used for testing many but not all tasks. It can test operations on the filesystem, it can test weird uses of subroutines, and maybe it can also test exporting a project.

I used GUT which is the most popular testing framework for Godot projects. The GUT config is a little weird because it includes the necessary configuration to search for tests together with personal customizations. I think you have to load the file manually the first time because it keeps the real configuration in your user data folder.

Though it's only intended as an example, it could also be merged as-is. I can write more tests like this if you give confirmation that you would use them and maintain them. Otherwise this can serve as an example of what kind of tests could be made in the future.