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/Cargo.toml

@@ -8,6 +8,7 @@ path = "lib.rs"
 
 [dependencies]
 anyhow = { workspace = true }
+assert_cmd = { workspace = true }
 codex-core = { workspace = true }
 serde_json = { workspace = true }
 tempfile = { workspace = true }