Add testing feature to validator_client/http_api (#8909)

Create a `testing` feature which we can use to gate off `test_utils.rs` and its associated dependencies from the rest of the crate.


Co-Authored-By: Mac L <mjladson@pm.me>

validator_manager/Cargo.toml

@@ -29,4 +29,4 @@ beacon_chain = { workspace = true }
 http_api = { workspace = true }
 regex = { workspace = true }
 tempfile = { workspace = true }
-validator_http_api = { workspace = true }
+validator_http_api = { workspace = true, features = ["testing"] }