Fix split_test xblock so that it can render in Studio

This change addresses STUD-1403 where split_test modules could not render in Studio. It changes Studio to pass a new 'runtime-type' of 'studio' when rendering an xblock. In split_test, this will then cause all of the children to be rendered, rather than just the one that matches the conditional.

Note: I had to bulletproof the code in split_test in case the user partitions are not available. I understand that this might not be the best long term solution, but hopefully this is tolerable for this week's release.

@dianakhuang @cpennington Please review.