This depends on what you're testing. If you're collapsing threads/processes into a single thread you're making decisions about the order of execution anyway so

This depends on what you're testing. If you're collapsing threads/processes into a single thread you're making decisions about the order of execution anyway so you're not going to catch errors introduced around unexpected preemption or inter-core process/thread timing issues. If you're not looking for that then you can build your software/system to have known sync-points where you can allow a given process to stop/wait and allow other processing to occur. This can then be in-process/out of process/remote. As you say, this ends up being a scheduler with which you have to employ knowledge about the execution/communication channels in order to coordinate correctly.