Fix `test_DISPPARAMS.TestCase.test`.

I think the following commented-out section might now succeed in its assertion.
If this test were to fail, tests for `IDispatch.Invoke` or dispatch methods would also fail as a result.

https://github.com/enthought/comtypes/blob/083c19e9fdc0f366df48ad448e897246f346ede0/comtypes/test/test_DISPPARAMS.py#L15-L26

```diff
@@ -15,12 +15,9 @@ class TestCase(ut.TestCase):
dp.rgvarg[1].value = "spam"
dp.rgvarg[2].value = "foo"

- # damn, there's still this old bug!
-
self.assertEqual(dp.rgvarg[0].value, 42)
- # these fail:
- # self.failUnlessEqual(dp.rgvarg[1].value, "spam")
- # self.failUnlessEqual(dp.rgvarg[2].value, "foo")
+ self.assertEqual(dp.rgvarg[1].value, "spam")
+ self.assertEqual(dp.rgvarg[2].value, "foo")

def X_test_2(self):
# basically the same test as above
```