Auto merge of #12398 - epage:cargo-config, r=weihanglo

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

This is split out of #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.

Author: bors