Allow for return types

[dzfrias]

Thanks for the great crate! This PR allows return types in test functions (namely for Result<T>).

[Y-Nak]

Thanks for this PR! The change looks good to me.
Would you modify a doc comment in dir-test/src/lib.rs so users can know this nice feature?

[Y-Nak]

Oh, no. I just remembered that rust doesn't allow the return type in the #[test] function.
Would you share the use case for it? We could easily extend this crate to generate a normal function which one-to-one corresponds to a file.

[dzfrias]

Rust allows for the Result type to be used in the return type of a test: link here. If the change is still okay, I'll add that doc comment!

[Y-Nak]

Ah, I'm embarrassed! I didn't know it's just required to implement std::process::Termination for the return type.

Alright, your change looks good to me! Thanks!

[dzfrias]

Okay, I added the relevant documentation!

[Y-Nak]

Thank you! I'll bump up the version.