Related to #886 .
Get rid of the magic constant 5000 here:
It should be dynamic and possibly discovered by the test itself at run-time.
Not possible without a mutex or semaphore or signal back from the subtask.
This test is still causing "out of order" failing tests on Travis:
[GH #907] skip task.t until rurban/task.t-gh907 is merged
Working on a semaphore implementation in rurban/task.t-gh907