tests/thread_pthread_barrier: Remove f string in test
tests/thread_pthread_barrier: Remove f string in test

This causes nightlies to fail as the HiL test runners don't have python3.6+

kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,ant9000/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,kYc0o/RIOT