Having `prev_now` initialized to 0 breaks tests on `arduino-zero` and `arduino-mega2560` as `xtimer_now_usec` is way bigger (72k on `arduino-zero`). Issue found in: * #9052 and proposed fix by ZetaR60 * https://github.com/RIOT-OS/Release-Specs/issues/62#issuecomment-387421737
Trickle Test
This test starts a trickle timer and roughly checks the diff between two
intervals to be greater than the diff of previous intervals.
After 5 callbacks, the trickle timer is reset and ends after another 7
callbacks with either [SUCCESS] or [FAILURE]. The application exits with
[FAILURE] as soon as one diff is not greater than the previous diff.