Add optional preprocessing before FileCheck

[tamird]

• [NFC] cargo fmt
• Add optional preprocessing before FileCheck

This allows us to implement tests in https://github.com/aya-rs/bpf-linker that invoke bpftool on our object files, and then run FileCheck on the output.

[tamird]

Thanks! @Manishearth could you cut a release with this?

[Manishearth]

Since it's a test dep are you okay depending on the git side for a while? I'm trying to not spend too much time maintaining this now that clippy doesn't use it.

[tamird]

Yep, no problem.