[OldMongo FC-0004] Tests for removing support for children in Old Mongo - part 4

[UvgenGen]

Description

Should be merged after: #31466
Fourth part of preparing tests for Remove support for children in Old Mongo task (#31134).

• updated test_import.py and test_course_index.py for split modulestore
• fixed 400 error for cms old_style assets

Useful information to include:
First part: #31422
Second part: #31427
Third part: #31466
Remove support for children in Old Mongo PR: #31134
openedx/public-engineering#80

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[mphilbrick211]

Hi @UvgenGen -is this ready for review?

[UvgenGen]

@mphilbrick211 yes, this is ready for review. I'll rebased it today.

[mphilbrick211]

Flagging this for you @ormsbee!

[ormsbee]

I'll merge this on Monday morning.

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EdX Release Notice: This PR has been deployed to the staging environment in preparation for a release to production.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the production environment.