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/src/user_notification.rs

@@ -20,7 +20,6 @@ pub(crate) enum UserNotification {
 
 #[cfg(test)]
 mod tests {
-    #![allow(clippy::unwrap_used)]
     use super::*;
 
     #[test]