There's nothing about rust that makes things hard to test. Actually the embedded test framework makes it easier than C. But what really matters is the public interface of those tools and that's got an extensive test suite available. It doesn't matter which language is used internal for those tests to run.
I meant that Rust is probably not a good choice for a new shell scripting environment, not that Rust is hard to test. I was responding to the claim "Unix shell environments have always been ad hoc and poorly tested", which is a bad thing and is worth fixing in and of itself.
