Exclude feature tests when uploading to crates.io

[figsoda]

When updating genact in nixpkgs (https://github.com/NixOS/nixpkgs/pull/191077), I ran into a hash mismatch for the cargo vendor because the feature-tests directory is absent on macos for some reason. I still don't know what is causing this issue, but It would be nice if the directory could be excluded in Cargo.toml.