fix: create separate test_support crates to eliminate #[allow(dead_code)] (#1667)

Because of a quirk of how implementation tests work in Rust, we had a
number of `#[allow(dead_code)]` annotations that were misleading because
the functions _were_ being used, just not by all integration tests in a
`tests/` folder, so when compiling the test that did not use the
function, clippy would complain that it was unused.

This fixes things by create a "test_support" crate under the `tests/`
folder that is imported as a dev dependency for the respective crate.

codex-rs/core/tests/common/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "core_test_support"
+version = { workspace = true }
+edition = "2024"
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+codex-core = { path = "../.." }
+serde_json = "1"
+tempfile = "3"
+tokio = { version = "1", features = ["time"] }