Update convert_to_capped_collection.js concurrency workload to support iteration multipliers

The convert_to_capped_collection.js workload will fail if you specify an iteration multiplier in a top-level runner file. It needs to be updated to support iteration counts that can cause `this.size` to become too small.

See: https://github.com/mongodb/mongo/blob/904c99891b47935ff7c0856f4456303f32c04f11/jstests/concurrency/fsm_workloads/convert_to_capped_collection.js#L15