Define a protocol to communicate between individual tests and nextest

[sunshowers]

Right now the only communication channel we have between the testrunner and Rust tests, during execution, is the process exit code. We'd like to have a richer protocol but the Rust test harness is kind of limited so we need to be careful about it. Some ideas for things to communicate:

• test flakiness
• test retries
• test timeouts

Some ideas for the protocol itself:

• Use lines to stdout (or stderr) that begin with nextest: (similar to cargo: lines in build scripts)
• Pass in a random string through the environment and ensure that lines begin with that to avoid confusing random test output with nextest control lines.
• Publish a crate which can be used to output these lines, either as function calls or as a proc macro.

[sunshowers]

Another solution pointed out by @yaahc: publish a Unix domain socket through an env var and use that to send control messages. That would work quite well and be a cleaner solution/less icky. Potential issue is that this will probably need to select on reading from the socket and stdout. Solutions would be:

• do the control channel and process monitoring on separate threads and send messages back and forth
• introduce unix-specific select code: https://docs.rs/nix/0.20.0/nix/sys/select/fn.select.html
• switch to tokio, which handles all this complexity for us at the cost of being a nontrivial dep