Similar to tenant migrations, we want to have passthrough tests which run continuous shard split operations in the background while running jsCore tests. Also consider whether we should add all flavors of tenant migration passthrough tests:

• jsCore
• stepdown
• kill primary
• multi stmt txn
• terminate primary
• causally consistent

This ticket covers the initial version of the passthrough tests which just run a vanilla jscore passthrough, and run a single split operation in the background.