Define a protocol to communicate between individual tests and nextest

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.

The issue names no files or tests. Start by reviewing the Rust test harness and nextest execution boundary, then compare the proposed stdout/stderr, environment-token, and crate approaches; done means an agreed protocol that supports richer test-run communication.

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