Add FileCheck mode

[clubby789]

This allows using, e.g. cargo-bisect-rustc --start 1.70.0 --filecheck src/lib.rs --preserve --regress success to investigate codegen regressions/improvements

[clubby789]

I'm not really sure why this changed

[apiraino]

These files are autogenerated by trycmd when running tests, based on the content of the cli options.

I forgot the details, but in tests/README.md I added some info a while ago

[clubby789]

I meant I'm not sure specifically why the formatting of the help output changed here - descriptions are now all placed on the following line

[apiraino]

I did some bisection of your patchset. The change that trigger this is the new command line argument

    #[arg(long, help = "Path to LLVM FileCheck")]
    filecheck_path: Option<PathBuf>,

if you remove the underscore from the parameter name (for example to filecheckpath), the help output does not add a newline anymore.

So, the issue is with clap. I quickly skimmed the changelog but couldn't find anything explaining this strange thing. Feel free to investigate further 😄

[clubby789]

Ah it's not the underscore, it's the name length - I think when the size of the longest arg name + the longest help string are such that having all arguments inline would be very long, it's wrapped round to a new line
Repro: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ba39c32190f89d5b28976c14b28e58cb
Remove the S from the argument name and it'll be in-line