Improve tests regarding permissions

[Sakib25800]

Fixes #212

[Sakib25800]

For PR #210, we could do something simple like:

fn create_world_with_delegate_config() -> World {
        let world = World::default();
        world.default_repo().lock().set_config(
            r#"
[labels]
approve = ["+approved"]
"#,
        );
        // Remove all permissions from PR author. 
        world
            .default_repo()
            .lock()
            .permissions
            .users
            .remove(&User::default());

        world
    }

This achieves the state we want.

PR #210 will probably need a few changes test wise, either via another PR or directly - whichever is suggested.

[Kobzol]

This is a nice clean-up, thanks. I think that the API could be still a bit nicer and more bulletproof, but that's for a larger refactoring. Also we should somehow check that the unprivileged user won't ever have any privileges in tests, but that might be tricky to verify, and it shouldn't happen by accident, so it should be fine.

Left one comment, otherwise LGTM.

[Kobzol]

Thanks!

[Kobzol]

Oh, this will need a rebase :)