Fix issue #3: validation error when encrypting large files.

* onetime
  (PadEncoder.encode): Don't record consumption in-loop, just record
    it once at the very end, so the reconsumption check is valid.

* check.sh: Print explanation of output at start of run.
  ("encryption, decryption of large plaintext"): Remove XFAIL label.
    Also, run faster, by iterating only 5 times instead of 10.  This
    makes the files half the size they used to be, but they're still
    easily large enough to stimulate the now-fixed bug.