tests/gnrc_netif: check if if_pid is set correctly in header
tests/gnrc_netif: check if if_pid is set correctly in header

Previously this was not tested and excepted KERNEL_PID_UNDEF == 0 as
legal value for received `gnrc_netif_hdr`s (which except for the
loopback interface is wrong)

aeneby/RIOT,smlng/RIOT,rfuentess/RIOT,yogo1212/RIOT,kYc0o/RIOT,toonst/RIOT,RIOT-OS/RIOT,ant9000/RIOT,x3ro/RIOT,aeneby/RIOT,jasonatran/RIOT,OTAkeys/RIOT,toonst/RIOT,cladmi/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,mtausig/RIOT,smlng/RIOT,josephnoir/RIOT,basilfx/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,mtausig/RIOT,yogo1212/RIOT,miri64/RIOT,authmillenon/RIOT,toonst/RIOT,smlng/RIOT,josephnoir/RIOT,x3ro/RIOT,kaspar030/RIOT,basilfx/RIOT,OTAkeys/RIOT,smlng/RIOT,OTAkeys/RIOT,kaspar030/RIOT,basilfx/RIOT,mtausig/RIOT,OlegHahm/RIOT,rfuentess/RIOT,aeneby/RIOT,x3ro/RIOT,x3ro/RIOT,rfuentess/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,smlng/RIOT,authmillenon/RIOT,ant9000/RIOT,yogo1212/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,jasonatran/RIOT,cladmi/RIOT,ant9000/RIOT,authmillenon/RIOT,x3ro/RIOT,OlegHahm/RIOT,josephnoir/RIOT,rfuentess/RIOT,basilfx/RIOT,toonst/RIOT,kYc0o/RIOT,yogo1212/RIOT,aeneby/RIOT,kaspar030/RIOT,yogo1212/RIOT,mtausig/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,authmillenon/RIOT,aeneby/RIOT,josephnoir/RIOT,toonst/RIOT,miri64/RIOT,ant9000/RIOT,cladmi/RIOT,cladmi/RIOT,jasonatran/RIOT,mtausig/RIOT,cladmi/RIOT,miri64/RIOT,basilfx/RIOT,rfuentess/RIOT