serde_test should be dev-dependency

[Mingun]

If I'm not missing something, it is required only for tests, so it should be in the [dev-dependencies] section

It is not required to made serde_test optional in dev-dependencies. It will be used only if some your code will depend on it, and this code will be included only when you enable the serde_support feature during tests. So if you run

cargo test --no-default-features

the serde_test dependency will not compiled (and even downloaded if I remember correctly) because you have no code that uses it.