reuse binary tests for prod builds

[Gankra]

It seems vaguely plausible that we could run Cargo's binary tests (/tests/...) for production binaries, which would be nice for allowing people to test prod binaries. This is really messy to do on github infra because it doesn't like "staging" releases (draft releases have different URLs from un-drafted ones, so anything that's self-downloading will be confused...), so it may require us to bring up our own infra to do Well.

[Gankra]

The integration tests now include an env var OVERRIDE_CARGO_BIN_EXE_cargo-dist=/path/to/binary which is checked at runtime and masks CARGO_BIN_EXE_cargo-dist. This allows you to inject an arbitrary binary from wherever into the test suite, and just run cargo test normally.

Remaining issue is to work out how to automate doing this on releases (ideally we'd somehow run the normal ci.yml test scripts, and not have to duplicate them...).

[Gankra]

Also note that this can theoretically be combined with nextest's support for archiving/cross-compiling tests: https://nexte.st/book/reusing-builds.html

I originally opened nextest-rs/nextest#917 to discuss this, and rain wrote a ton of insightful notes (culminating in me being convinced that all I needed for now was the OVERRIDE_* pattern, and that no upstream work is necessary).