fix: enable clippy on tests (#870)

https://github.com/openai/codex/pull/855 added the clippy warning to
disallow `unwrap()`, but apparently we were not verifying that tests
were "clippy clean" in CI, so I ended up with a lot of local errors in
VS Code.

This turns on the check in CI and fixes the offenders.

codex-rs/core/src/is_safe_command.rs

@@ -194,6 +194,7 @@ fn is_valid_sed_n_arg(arg: Option<&str>) -> bool {
 }
 #[cfg(test)]
 mod tests {
+    #![allow(clippy::unwrap_used)]
     use super::*;
 
     fn vec_str(args: &[&str]) -> Vec<String> {