Support specifying target for test cases

[upsuper]

Some proc macros may want to ensure that they work properly for targets with only no_std supported, for example, as Rust 1.64 stabilizing core::ffi::CStr, cstr can soon work in no_std, and it would be good if there is a way to test that.

One possible solution is to make it possible to specify target on a TestCases, for example,

impl TestCases {
    pub fn with_target(target: &'static str) -> Self;
}

and then create a .cargo/config.toml in the generated test directory.

What do you think?

Also if you are busy, I can work on this and raise a PR as long as you are fine with this feature.

[upsuper]

Actually... if we are cross compiling, we may also need a way to have build-only tests (in addition to pass and compile_fail), since we can't run executable in a different target.

[joshlf]

I would also like to see a feature like this for zerocopy-derive (a proc macro) so that even if I'm running my tests on a particular host (e.g., the Linux VMs supported by GitHub Actions), I can still make sure that zerocopy-derive produces correct code when the build target is a different architecture. This would entail trybuild invoking the compiler and instructing it to compile for a different target.

[dtolnay]

I think I would prefer not to build this into this crate, but it would be reasonable for someone else to maintain a more fully featured fork that handles this.