tests/trickle: initialize prev_now in main

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