Printing can take a lot of time over a serial connection. On the atmega
board it takes more than 2 ms, skewing the first measurement over the
threshold value and causing a test failure. This moves the print
statement above the first measumerement to prevent the skew.
This test measures the difference of two consecutive calls to xtimer_now64()
100k times. If the max or average difference is larger than 1000us the test
fails, otherwise it succeeds.
