The CMAP spec test runner currently does some pre-processing to delegate threaded operations to individual threads ahead of time, and they are executed one after another after the associated thread is started via a start operation.

The spec tests assume that each operation, threaded or not, happens in the order that they're specified, with threaded operations being delegated to the respective threads. We should update the CMAP runner to behave in this manner.

Context: https://github.com/mongodb/specifications/pull/882#discussion_r534399397