The following class is serialized differently when using `bson-kotlin` and `bson-kotlinx`:

@Serializable
data class U(
    val x: kotlin.time.Instant,
)

With `bson-kotlin`, the field `x` is correctly serialized as a DateTime.

However, when using `bson-kotlinx`, it is serialized as a String.

This means that this class doesn't pass the Datetime tests.