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/diff_render.rs

@@ -357,7 +357,6 @@ fn style_del() -> Style {
     Style::default().fg(Color::Red)
 }
 
-#[allow(clippy::expect_used)]
 #[cfg(test)]
 mod tests {
     use super::*;