Add raise_exception() to the _testcapi module.  It isn't a test, but
the C API exists only to support test_exceptions.  raise_exception()
takes two arguments -- an exception class and an integer specifying
how many arguments it should be called with.

test_exceptions uses BadException() to test the interpreter's behavior
when there is a problem instantiating the exception.  test_capi1()
calls it with too many arguments.  test_capi2() causes an exception to
be raised in the Python code of the constructor.