tests: add test for Rust formatting We enforce formatting for Python and C. It makes sense to do it for Rust as well. Since our rustfmt.toml relies on unstable rustfmt features, we need to use a Nightly rustfmt with --unstable-features in order for it to work. This is a bit hacky and I would prefer we remove this requirement. But for now, this commit assumes this is the way things must be and we go out of our way to detect and use the rustfmt from the "nightly" toolchain, as installed via rustup. We had to add some environment variables to the tests to make the Rust binaries happy. Otherwise when running rustfmt we get an error about no default toolchain being installed. Differential Revision: https://phab.mercurial-scm.org/D7579

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