before:
```
$ time cargo test -p codex-tui -q
[...]
cargo test -p codex-tui -q 39.89s user 10.77s system 98% cpu 51.328 total
```
after:
```
$ time cargo test -p codex-tui -q
[...]
cargo test -p codex-tui -q 1.37s user 0.64s system 29% cpu 6.699 total
```
the major offenders were the textarea fuzz test and the custom_terminal
doctests. (i think the doctests were being recompiled every time which
made them extra slow?)
bbf42f4e12