When merging configuration lists, the current order does not match
the expected precedence. This makes merged lists follow precedence
order, with higher precedence items merged later in lists.
This mirrors some of the categories from `cargo help` (the man pages).
There are fewer categories to avoid extra vertical space. Instead, they
are left int the `Options` category but put first.
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.
