Currently the lint faces a severe limitation: since it only catches single-line block, running rustfmt beforehand will remove all occurences of it, because it breaks them into multiline blocks.

We do not check match `Arm` for two reasons:
- In case it does not use commas to separate arms, removing the block would result in a compilation error
Example:
```
    match expr {
        pat => {()}
        _ => println!("foo")
    }
    ```
    - Do not lint multiline match arms used for formatting reasons
    ```
    match expr {
       pat => {
           somewhat_long_expression
       }
     // ...
    }
```

Delete `unused-braces-lint` test

The modified lint correctly provide a span in its suggestion.

```shell
    error: unnecessary braces around block return value
      --> /rust/src/test/rustdoc-ui/unused-braces-lint.rs:9:5
       |
    LL | /     {
    LL | |         {
       | |________^
    LL |               use std;
    LL |           }
       |  __________^
    LL | |     }
       | |_____^
       |
    note: the lint level is defined here
      --> /rust/src/test/rustdoc-ui/unused-braces-lint.rs:6:9
       |
    LL | #![deny(unused_braces)]
       |         ^^^^^^^^^^^^^
    help: remove these braces
       |
    LL ~     {
    LL |             use std;
    LL ~         }
       |
```

It is unclear to which extend rust-lang#70814 is still an issue, as the inital MCVE does not trigger the lint on stable either,[rust playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b6ff31a449c0b73a08daac8ee43b1fa6)

Fix code with expanded `unused_braces` lint

Also allow `unused_braces` on tests

Mute `unused_braces` on `match_ast!`