Getting draft split_test_module instances to import/export.

STUD-327

@cpennington and @nasthagiri Please review my changes to make draft split_test_module instances import correctly. ImportSystem for published content is created with the source course ID, but for draft content it was being created with the target course ID. Also fixed some issues with assumptions about draft verticals being nested immediately below sequentials.

I will squash commits if these changes look good. The first commit explains the issue with passing the target course ID to ImportSystem.

The majority of the files in the PR are for a test course used by the CMS unit test. I thought of adding this new test course to the roundtrip import/export test, but it didn't catch the original bugs, so doesn't seem worthwhile. However, while trying that out I found the bug of xml_attributes not calling serialize_field.

Ah, I see.

Another possible issue: can split_tests be drafts, and then not be imported by this code (which is only looping over verticals)? Or, more generally, will this code fail if there's something other than a vertical at the level of a unit?