The scaled_collection_stats.js test inserts a bunch of documents to a collection, then determines the unscaled total index size and finally checks that the totalIndexSize scaled by itself (as read in a previous command) equals to 1.

The totalIndexSize metric represents the index size on disk, rather than on memory, so the test needs to ensure that the collection data has been flushed to disk. This is similar to SERVER-28716.

This can be done by issuing a flush command (e.g. like in this test).