Add codex exec testing helpers (#4254)

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

codex-rs/exec/tests/suite/mod.rs

@@ -1,6 +1,5 @@
 // Aggregates all former standalone integration tests as modules.
 mod apply_patch;
-mod common;
 mod output_schema;
 mod resume;
 mod sandbox;