fifo: 8 x 8-bit synchronous FIFO
  ok    empty after reset = 1
  ok    count after reset = 0
  ok    count when full = 8
  ok    full flag = 1
  ok    empty when full = 0
  ok    count, push when full = 8
  ok    empty after draining = 1
  ok    count after draining = 0
  ok    count, pop when empty = 0
  ok    empty after wrapping = 1
PASS  fifo
