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/linux.rs

@@ -179,7 +179,9 @@ fn install_network_seccomp_filter_on_current_thread() -> std::result::Result<(),
 }
 
 #[cfg(test)]
-mod tests_linux {
+mod tests {
+    #![allow(clippy::unwrap_used)]
+
     use super::*;
     use crate::exec::ExecParams;
     use crate::exec::SandboxType;