Feasibility tests can not be run with sporadic task

[D. Orfanos]

People who would like to run feasibility tests on such task types must do it by modeling them with periodic tasks.

Such a modeling of sporadic tasks by periodic tasks is a way to model the worst case of processor consumption of sporadic tasks.

This fix forbids the use of sporadic tasks with feasibility tests
(some MMI messages was fixed with this ticket).