Cargo test json output may change

There are grumblings about a renewed effort to stabilize cargo's json output. This is technically great news for us, as we'll be able to use stable rust (and make the dockerfile much simpler).

The downside is that there's a good chance the format will change.

The current development can be read here:
https://internals.rust-lang.org/t/path-for-stabilizing-libtests-json-output/20163/4