The resharding_oplog_sync_agg_assert_min_oplog.js test intentionally uses a small oplog size (1MB) to force oplog truncation to occur without needing to insert a large amount of data. The implicit of w=1 write concern allows the replica set secondary member to fall behind and be unable to catch up from the primary as a sync source. However, it isn't necessary when asserting properties of the $_requestReshardingResumeToken option to the aggregate command to even have a replica set secondary member.