arv-test: don't display snapshot option if cairo is not available.
arv-test: don't display snapshot option if cairo is not available.

AnilRamachandran/aravis,AravisProject/aravis,lu-zero/aravis,lu-zero/aravis,lu-zero/aravis,AnilRamachandran/aravis,AnilRamachandran/aravis,AnilRamachandran/aravis,AravisProject/aravis,lu-zero/aravis,AnilRamachandran/aravis,AravisProject/aravis,AravisProject/aravis,lu-zero/aravis,AravisProject/aravis