Test can fail because of the formula being used to determine which chunks to move:

https://github.com/mongodb/mongo/blob/7acafe85d9bdd63122c19ba1cca86a7f55174941/jstests/sharding/migrateMemory.js#L47

Depending on how the chunks (specifically, the number of chunks) are actually split*, it is possible to have 2 consecutive numbers that are actually bounded by the same chunk, resulting into the "that chunk is already on that shard" error. We should just remove this test since it has no useful asserts.

* Our internal chunk stats for stats are initialized with a random number so the point when it triggers the auto split is not deterministic.