Fix testsuite RTPOSIXSolver

Update regular expression for new error message from RTPOSIXSolver:

rtposixsolver.c:

silent_cerr("RTPOSIXSolver: sched_setscheduler failed (" << save_errno << ": " << strerror(save_errno) << ")" << std::endl);

parse_mbdyn_error_message.awk:

/^RTPOSIXSolver: sched_setscheduler failed \(1: .*\)$/ { set_error("feature", "RTPOSIX"); }