Fixed three exceptions in the Plain integers test, although I'm not

Fixed three exceptions in the Plain integers test, although I'm not
sure these are the best fixes.

- Test maxint-1 against the negative octal constant -020000000000

- Comment out the tests for oct -1 and hex -1, since 037777777777 and
  0xffffffff raise FutureWarnings now and in Python 2.4 those
  constants will produce positive values, not negative values.  So the
  existing test seems to test something that won't be true in 2.4.