refactor(test): Move cargo-config into a dir

This is split out of rust-lang#11912 and is prep for adding more UI tests.

Generally our UI tests are in a directory named after the full cargo
command (`cargo config`).  These tend to use `snapbox`.

Here we are tests for the `cargo config` command not written by
`snapbox` in a `cargo_config.rs` file.  This conflicts with adding
snapbox UI tests later in a `cargo_config/` folder.  Upon looking at this
file, it appears to be UI tests, so I think it would make sense to move
them into the `cargo_config/` folder.  Definitely wouldn't make sense to
move them into `config.rs` since that is general config testing.