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:
* RIOT-OS#9052 and proposed fix by ZetaR60
* RIOT-OS/Release-Specs#62 (comment)

tests/trickle/main.c

@@ -60,6 +60,7 @@ int main(void)
 {
     msg_t msg;
     unsigned counter = 0;
+    prev_now = xtimer_now_usec();
 
     trickle_start(sched_active_pid, &trickle, TRICKLE_MSG, TR_IMIN,
                   TR_IDOUBLINGS, TR_REDCONST);