I used this to test

var arr1 = [ [ 1, 2, 3, 4, 5 ],
  [ 6, 7, 8, 9, 10 ],
  [ 11, 12, 13, 14, 15 ],
  [ 16, 17, 18, 19, 20 ],
  [ 21, 22, 23, 24, 25 ] ]

Test.assertSimilar(snail(arr1), [1, 2, 3, 4, 5, 10, 15, 20, 25, 24, 23, 22, 21, 16, 11, 6, 7, 8, 9, 14, 19, 18, 17, 12, 13], 'Hmm')