* deterministic.
 *
 * This class ensures that at most one thread is running at a time. This is initially the JUnit test
 * thread, which yields its execution privilege while calling [runTasks], [runNextTask], or
 * [advanceUntil]. These functions don't return until the task threads are all idle.
 *
 * Task threads release their execution privilege in these ways:
 *
 *  * By yielding in [TaskRunner.Backend.coordinatorWait].

    // Running the red task starts another thread, so the two can run in parallel.
    taskFaker.runNextTask()
    assertThat(log).containsExactly("red:starting@0")
    assertThat(taskFaker.executeCallCount).isEqualTo(2)

    // Next the blue task starts.
    taskFaker.runNextTask()
    assertThat(log).containsExactly(
      "red:starting@0",
      "blue:starting@0",
    )