The create_index_background_unique_collmod.js and create_index_background_unique_collmod_capped.js workloads (added in SERVER-41974) presently do a large number of empty collMod commands in order to trigger the unique index upgrade code path during unique index creation and deletion.

Too many of these collMods in succession can starve resources and cause the test to time out.

We can address this problem by reducing the probability of the collMod executing during the workload.