ui_test_core: do not sleep in Zuite_waitForActivities
Otherwise in the worst case we sleep for 1000 seconds.
This is too aggressive if we have activities that are delayed. Besides activity errors (I admit we should not have any), code can for example use at_date to throttle execution for an interface (we do have this in a project); this may not be pertinent enough for UI tests. In non-UI tests, the clock is shifted by 45s every 10 iterations.