Function addExecutor
Adds the executor named name to the LifeCycle listeners collection
Example
addExecutor throws for the unknown executor "nope"
auto old = LifeCycleListeners .instance;
LifeCycleListeners .instance = new LifeCycleListeners;
scope (exit) LifeCycleListeners .instance = old;
({ addExecutor("nope", Settings()); }) .should .throwException!Exception .withMessage("There is no `nope` executor");