Nicer partest formatting of 4-digit test numbers

Because

  ...
  [partest] ok  999 - pos/t3534.scala
  [partest] ok 1000 - pos/t261-ba.scala
  ...

is so much nicer than

  ...
  [partest] ok 999 - pos/t3534.scala
  [partest] ok 1000 - pos/t261-ba.scala
  ...