Add codex exec testing helpers (#4254)

Add a shortcut to create working directories and run codex exec with
fake server.

codex-rs/core/tests/common/lib.rs

@@ -9,6 +9,7 @@ use codex_core::config::ConfigToml;
 
 pub mod responses;
 pub mod test_codex;
+pub mod test_codex_exec;
 
 /// Returns a default `Config` whose on-disk state is confined to the provided
 /// temporary directory. Using a per-test directory keeps tests hermetic and