Allow building helper binaries for tests

I've written a few crates that do process manipulation, and when writing tests I find myself needing a helper binary to run and manipulate. Three quick examples are read-process-memory, spawn-ptrace, and spawn-task-port.

I currently just put those helper binaries in src/bin so they get built as programs, but they're not intended for end-user consumption and don't need to be built except for test usage. It would be nicer if cargo had a way to build helper binaries for tests, like perhaps .rs files under tests/bin could be treated as such? Helper binaries should be considered a dependency of running tests, so they should be rebuilt as necessary before tests are run, but could be built in parallel with the tests themselves.

Start by reviewing the existing helper-binary examples in src/bin/test.rs from read-process-memory, spawn-ptrace, and rust-spawn-task-port. Determine how Cargo could discover and build test-only binaries, including rebuilding them before tests and allowing parallel execution; done means this workflow is supported without treating helpers as end-user binaries.