use a file that should always exist
use a file that should always exist

That test relied upon a `README.md` in the exercise directory. The
current problem specification deprecated that. So I changed the test to
assert against `Cargo.toml` which I believe should always be there.

petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust