tests/xtimer_drift: increase priority of worker

When the slacker threads and the worker thread have the same priority
the drift and jitter depend on the workload.
Timer set before the worker thread will be fired before the worker and so
the result is not the timings the system might reach when proper priority
is set. Increasing the priority of the worker thread ensures that the
measureed timings are the achievable timings.

tests/xtimer_drift/main.c

@@ -163,7 +163,7 @@ int main(void)
 
     /* create and trigger worker thread */
     kernel_pid_t pid3 = thread_create(worker_stack, sizeof(worker_stack),
-                                      THREAD_PRIORITY_MAIN - 1,
+                                      THREAD_PRIORITY_MAIN - 2,
                                       THREAD_CREATE_STACKTEST,
                                       worker_thread, NULL, "worker");