New test function cmd_test_xtimer_mutex_lock_timeout_long_locked.
In this test the mutex is locked and the timeout is long.
When it works the thread continues running and stops waiting for the mutex and
the function will return that it did not get the mutex.
Adding a first normal test case where the mutex is unlocked and the timeout is long.
The timer will not trigger in this test and instead wil be removed after getting the mutex.
