`cargo test` for the core and other crates fails on a fresh macOS
checkout without the right stack size variable. This change encourages
using the just test command that sets the environment up correctly.
As a bonus, this should encourage agents to get more benefit out of
nextest's parallel execution.
Enhance pty utils:
* Support closing stdin
* Separate stderr and stdout streams to allow consumers differentiate them
* Provide compatibility helper to merge both streams back into combined one
* Support specifying terminal size for pty, including on-demand resizes while process is already running
* Support terminating the process while still consuming its outputs
