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/tests/head.rs

@@ -67,7 +67,10 @@ fn test_head_one_flag_one_file() -> Result<()> {
             exec: ValidExec {
                 program: "head".to_string(),
                 flags: vec![],
-                opts: vec![MatchedOpt::new("-n", "100", ArgType::PositiveInteger).unwrap()],
+                opts: vec![
+                    MatchedOpt::new("-n", "100", ArgType::PositiveInteger)
+                        .expect("should validate")
+                ],
                 args: vec![MatchedArg::new(
                     2,
                     ArgType::ReadableFile,

codex-rs/execpolicy/tests/sed.rs

@@ -47,7 +47,10 @@ fn test_sed_print_specific_lines_with_e_flag() -> Result<()> {
             exec: ValidExec {
                 program: "sed".to_string(),
                 flags: vec![MatchedFlag::new("-n")],
-                opts: vec![MatchedOpt::new("-e", "122,202p", ArgType::SedCommand).unwrap()],
+                opts: vec![
+                    MatchedOpt::new("-e", "122,202p", ArgType::SedCommand)
+                        .expect("should validate")
+                ],
                 args: vec![MatchedArg::new(3, ArgType::ReadableFile, "hello.txt")?],
                 system_path: vec!["/usr/bin/sed".to_string()],
             }