drop_index.js fails when run in passthrough suite with stepdown

When the jsCore test drop_index.js is run in a replica set passthrough configuration, a stepdown occurring after dropping multiple indexes may result in the subsequent listIndexes command returning one of the dropped indexes. This could be caused by the listIndexes connected to a new primary that may not replicated the oplog entries for both dropped indexes (c_1 and d_1).

According to SERVER-37727, a successful dropIndexes command from the client for multiple indexes results in replicating one dropIndexes oplog entry per index dropped.