tests: fix mutex_unlock_and_sleep pids

The `thread_wakeup(...)` calls in `main.c` of `tests/mutex_and_sleep` go to PID `1` and `2`, which are the `idle` thread and `main` (`main` tries to wakeup itself and `second_thread` tries to wake up the idle thread).
This PR fixes that by making `main` wake up `second_thread` and `second_thread` wake up `main`.
I've tested `native` and `arduino-mega2560`.