Require FEATURE_SERIALIZABLE_VALUES for SerializeCustomId tests

I am not 100% certain this is the right fix. But a PR seems like the simplest way to arrange any needed discussion.

The question is, if you `supportsUserSuppliedIds`, what data types are allowed for those ids? I am pretty certain that the intention isn't to require any Java data type. Perhaps the corresponding property features is a logical choice. That's what I've done here. But I could well imagine a database with stricter requirements. For example, a `Double` key is a recipe for trouble; among other things if you make one with `NaN` you may or may not be able to find it again, depending on the details of the implementation. Still, a whole new `DataTypeFeatures` for user-supplied ids feels like overkill.