tests/cpp11_thread: use sched_num_threads instead of const 2
tests/cpp11_thread: use sched_num_threads instead of const 2

When CDC ACM is used as stdio the first thread in the test may have a
different ID than #3.

kaspar030/RIOT,OlegHahm/RIOT,basilfx/RIOT,jasonatran/RIOT,basilfx/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,basilfx/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OlegHahm/RIOT,basilfx/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,miri64/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kaspar030/RIOT