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/tui/src/app.rs

@@ -188,7 +188,7 @@ impl App<'_> {
                         }
                     }
 
-                    #[allow(clippy::expect_used)]
+                    #[expect(clippy::expect_used)]
                     let deadline = next_deadline.expect("deadline set");
                     let now = Instant::now();
                     let timeout = if deadline > now {