Failed CI run:
https://github.com/zed-industries/zed/actions/runs/25559568951/job/75027378760?pr=56181

test_kill_active_task_on_completed_task_is_noop was flaking on Linux.
It's sibling was just waiting 200ms for PTY events to be handled by
alacritty. This PR polls the PTY instead of waiting an arbitrary amount
of time

Release Notes:

- N/A