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 parameter '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.
Merge pull request #2924 from edx/andya/studio-split-test-support-2

Fix split_test xblock so that it can render in Studio