I suggest adding values like 19999999 to avoid some brute-force approaches, e.g.:

Test.it("Bigger numbers",function(){
  Test.assertEquals(nextBigger(19999999),99999991)
  //...
})

Run it with 19999999 and crack some eggs on your macbook because this baby's gonna get pretty hot!

(actually skip one 9 as this one takes ca. 70sec on my machine)

Seems like the perfomance impact is exponential for cases like this (someone correct me if I'm wrong here).