testDispatcher: use idle_add not setTimeout
testDispatcher: use idle_add not setTimeout

Because setTimeout will add things with a higher priority than the
actual dispatcher idle, its not actually appropriate for these tests.
[endlessm/eos-sdk#4095]

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib