[Test-only bug]
On slow variants, the top-level chunk migration can require a lot of time and basically invalidates the purpose of the test (that is making sure that bulk writes and balancer are working together).

The flow for slow variants is the following:

• Chunk splitter begins move chunk
• Balancer is started
• Balancer makes some NOOP rounds
• Balancer is stopped
• Chunk splitter's move chunk finishes [or the test times out]