Many Shard Merge tests currently use the test framework's enableDonorStartMigrationFsync flag. When this flag is set, the donor executes an fsync before the migration begins. Thus, if we wrote data to the donor before the migration, that data appears in the latest snapshot and it's copied to the recipient via file-copy.

If we don't call fsync, then donor data usually doesn't appear in the snapshot. It's copied to the recipient via oplog application instead.

We need to thoroughly test both mechanisms for migrating data. Everywhere we use enableDonorStartMigrationFsync, let's test both with and without enableDonorStartMigrationFsync, if possible.