Fix suppressed error due to enhanced error message of RTPOSIX solver.

The usual outcome of running that test without being superuser was

reading from file "pendulum" RTPOSIXSolver: sched_setscheduler failed An error occurred during the execution of MBDyn; aborting...

Now it is

reading from file "pendulum" RTPOSIXSolver: sched_setscheduler failed (1: Operation not permitted) An error occurred during the execution of MBDyn; aborting...