'L{util._replaceIf} swaps out the body of a function if the conditional
is C{True}.'

def test_replacesIfTrue(self):

    @util._replaceIf(True, (lambda : 'hi'))
    def test():
        return 'bye'
    self.assertEqual(test(), 'hi')
    self.assertEqual(test.__name__, 'test')
    self.assertEqual(test.__module__, 'twisted.python.test.test_util')
