Fix: Fixed the broken integration tests after the removal of old Mongo

[Anas12091101]

What are the relevant tickets?

https://github.com/mitodl/rapid-response-xblock/issues/142

What's this PR do?

This PR fixes the integration tests which were broken after the removal of old Mongo support for parent child relationship Our tests started to break after the https://github.com/openedx/edx-platform/pull/31134 was merged giving us an ScopeError This PR changes the tests to use the new SplitMongo modulestore

See more information in the relevant issue ticket

How should this be manually tested?

• Clone the https://github.com/mitodl/rapid-response-xblock/commit/31bf3b89abbd730cd458a3323d259f6ae89c975b in the src folder of your devstack work directory
• Open the terminal and run ./run_devstack_integration_tests.sh
• All the tests should pass