I am afraid you cannot. The dataset API is a way to efficiently stream inputs to your net at run time. It is not a set of tools to manipulate datasets as a whole -- in that regards it might be a bit of a misnomer.
Also, if you could, this would probably be a bad idea. You would rather have this train/test split done once and for all.
- it let you review those sets offline
- if the split is done each time you run an experiment there is a risk that samples start swapping sets if you are not extremely careful (e.g. when you add more data to your existing dataset)
See also a related question about how to split a set into training & testing in tensorflow.