Problem: closing the test editor left cp://test-N-* buffers alive,
causing E95 on reopen. The nofile buftype also rejected :w, which
was counterintuitive in an editable grid.
Solution: delete all test buffers in toggle_edit teardown. Switch
buftype to acwrite with a BufWriteCmd autocmd that persists test
cases and clears the modified flag. Hoist save_all_tests above
setup_keybindings so the autocmd closure can reference it.
