Provide a task scheduler to simplify the tests.

Currently, there are a lot of tests that just want to launch a task in order to test some behavior of the system. These tests have to create their own v0 or v1 scheduler and invoke the necessary calls on it and expect the necessary calls / messages back. This is rather verbose.

It would be helpful to have some better abstractions here, like a TestScheduler that can launch tasks and exposes the status updates for them, along with other interesting information. E.g.

class TestScheduler
{
  // Add the task to the queue of tasks that need to be launched.
  // Returns the stream of status updates for this task.
  Queue<StatusUpdate> addTask(const TaskInfo& t);

  etc
}

Probably this could be implemented against both v0 and v1, if we want to parameterize the tests.