Added allow-expect-in-tests / allow-unwrap-in-tests (#2328)

This PR:
* Added the clippy.toml to configure allowable expect / unwrap usage in
tests
* Removed as many expect/allow lines as possible from tests
* moved a bunch of allows to expects where possible

Note: in integration tests, non `#[test]` helper functions are not
covered by this so we had to leave a few lingering `expect(expect_used`
checks around

codex-rs/core/tests/stream_no_completed.rs

@@ -33,8 +33,6 @@ fn sse_completed(id: &str) -> String {
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn retries_on_early_close() {
-    #![allow(clippy::unwrap_used)]
-
     if std::env::var(CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR).is_ok() {
         println!(
             "Skipping test because it cannot execute when network is disabled in a Codex sandbox."