Reworked test_warnings.py:

* It ran fine under "python regrtest.py test_warnings" but failed under
  "python regrtest.py" presumably because other tests would add to
  filtered warnings and not reset them at the end of the test.

* Converted to a unittest format for better control.  Renamed
  monkey() and unmonkey() to setUp() and tearDown().

* Increased coverage by testing all warnings in __builtin__.

* Increased coverage by testing regex matching of specific messages.