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/execpolicy/tests/suite/bad.rs

@@ -0,0 +1,9 @@
+use codex_execpolicy::NegativeExamplePassedCheck;
+use codex_execpolicy::get_default_policy;
+
+#[test]
+fn verify_everything_in_bad_list_is_rejected() {
+    let policy = get_default_policy().expect("failed to load default policy");
+    let violations = policy.check_each_bad_list_individually();
+    assert_eq!(Vec::<NegativeExamplePassedCheck>::new(), violations);
+}