`PublisherWorkflow` in WorkflowCombine breaks workers attempting to use any scheduler besides main.

[mjohnson12]

PublisherWorkflow is using subscribe(on:) which is forcing all the publishers to use runloop main to run.
In unit tests it's useful to support test schedulers so you can fast forward time on the scheduler. Useful if testing timers.
Using a Scheduler besides main currently doesn't work.

[mjohnson12]

Fixed.