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/execpolicy/src/execv_checker.rs

@@ -140,6 +140,7 @@ fn is_executable_file(path: &str) -> bool {
 
 #[cfg(test)]
 mod tests {
+    #![allow(clippy::unwrap_used)]
     use tempfile::TempDir;
 
     use super::*;