Adds an integration test for the new behavior introduced in
https://github.com/openai/codex/pull/9011. The work to create the test
setup was substantial enough that I thought it merited a separate PR.
This integration test spawns `codex` in TUI mode, which requires
spawning a PTY to run successfully, so I had to introduce quite a bit of
scaffolding in `run_codex_cli()`. I was surprised to discover that we
have not done this in our codebase before, so perhaps this should get
moved to a common location so it can be reused.
The test itself verifies that a malformed `rules` in `$CODEX_HOME`
prints a human-readable error message and exits nonzero.
this dramatically improves time to run `cargo test -p codex-core` (~25x
speedup).
before:
```
cargo test -p codex-core 35.96s user 68.63s system 19% cpu 8:49.80 total
```
after:
```
cargo test -p codex-core 5.51s user 8.16s system 63% cpu 21.407 total
```
both tests measured "hot", i.e. on a 2nd run with no filesystem changes,
to exclude compile times.
approach inspired by [Delete Cargo Integration
Tests](https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html),
we move all test cases in tests/ into a single suite in order to have a
single binary, as there is significant overhead for each test binary
executed, and because test execution is only parallelized with a single
binary.
