tests/trickle: initialize prev_now in main
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

basilfx/RIOT,yogo1212/RIOT,neiljay/RIOT,x3ro/RIOT,lazytech-org/RIOT,avmelnikoff/RIOT,biboc/RIOT,x3ro/RIOT,aeneby/RIOT,smlng/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,gebart/RIOT,mtausig/RIOT,miri64/RIOT,BytesGalore/RIOT,smlng/RIOT,josephnoir/RIOT,basilfx/RIOT,neiljay/RIOT,OTAkeys/RIOT,miri64/RIOT,toonst/RIOT,miri64/RIOT,lazytech-org/RIOT,ant9000/RIOT,avmelnikoff/RIOT,A-Paul/RIOT,mfrey/RIOT,smlng/RIOT,toonst/RIOT,josephnoir/RIOT,cladmi/RIOT,BytesGalore/RIOT,neiljay/RIOT,gebart/RIOT,A-Paul/RIOT,OTAkeys/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,kaspar030/RIOT,aeneby/RIOT,x3ro/RIOT,smlng/RIOT,mfrey/RIOT,basilfx/RIOT,kYc0o/RIOT,biboc/RIOT,ant9000/RIOT,BytesGalore/RIOT,miri64/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,mtausig/RIOT,toonst/RIOT,rfuentess/RIOT,josephnoir/RIOT,BytesGalore/RIOT,neiljay/RIOT,avmelnikoff/RIOT,mtausig/RIOT,smlng/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,avmelnikoff/RIOT,OTAkeys/RIOT,biboc/RIOT,kbumsik/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,gebart/RIOT,kbumsik/RIOT,jasonatran/RIOT,lazytech-org/RIOT,cladmi/RIOT,A-Paul/RIOT,authmillenon/RIOT,mtausig/RIOT,rfuentess/RIOT,cladmi/RIOT,cladmi/RIOT,mfrey/RIOT,gebart/RIOT,yogo1212/RIOT,kYc0o/RIOT,gebart/RIOT,kbumsik/RIOT,jasonatran/RIOT,biboc/RIOT,basilfx/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,mtausig/RIOT,neiljay/RIOT,authmillenon/RIOT,x3ro/RIOT,toonst/RIOT,yogo1212/RIOT,yogo1212/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,yogo1212/RIOT,toonst/RIOT,rfuentess/RIOT,kbumsik/RIOT,OTAkeys/RIOT,x3ro/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,kYc0o/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,aeneby/RIOT,biboc/RIOT,rfuentess/RIOT,cladmi/RIOT,authmillenon/RIOT,jasonatran/RIOT,aeneby/RIOT,jasonatran/RIOT,aeneby/RIOT,mfrey/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,A-Paul/RIOT,OlegHahm/RIOT,A-Paul/RIOT,ant9000/RIOT,kbumsik/RIOT,josephnoir/RIOT,yogo1212/RIOT,miri64/RIOT,mfrey/RIOT