Currently, the test for defragmentation (balancer_defragmentation_merge_chunks.js) is testing that defragmentation has happened solely based on the number of chunks. This is not a very robust or thorough and should be replaced by checks on the chunks existing after the tests run.

After phase 1, we should ensure that there are no sequential chunks on the same shard. In phase 2, we can check that there are no small chunks left who could be merged with another chunk. And for phase 3, we should check that there are no chunks over the configured maxChunkSize.