Merge pull request #3786 from kaspar030/fix_tests_xtimer_usleep_until

tests: xtimer_usleep_until: fix diff calculation

tests/xtimer_usleep_until/main.c

@@ -37,7 +37,7 @@ int main(void)
         uint32_t last_wakeup = xtimer_now();
         uint32_t before = last_wakeup;
         xtimer_usleep_until(&last_wakeup, (unsigned)interval);
-        uint32_t diff = (last_wakeup-before)-interval;
+        uint32_t diff = (xtimer_now()-before)-interval;
         res[i] = diff;
         interval -= 1;
     }