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/mcp-server/Cargo.toml

@@ -37,6 +37,7 @@ uuid = { version = "1", features = ["serde", "v4"] }
 
 [dev-dependencies]
 assert_cmd = "2"
+mcp_test_support = { path = "tests/common" }
 pretty_assertions = "1.4.1"
 tempfile = "3"
 tokio-test = "0.4"