test: faster test execution in codex-core (#2633)

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.

codex-rs/core/tests/suite/mod.rs

@@ -0,0 +1,12 @@
+// Aggregates all former standalone integration tests as modules.
+
+mod cli_stream;
+mod client;
+mod compact;
+mod exec;
+mod exec_stream_events;
+mod live_cli;
+mod prompt_caching;
+mod seatbelt;
+mod stream_error_allows_next_turn;
+mod stream_no_completed;