winhttp/tests: Fix test error when run as non-administrator

[Juan Lang]

Hi Paul,

> I agree that ERROR_ACCESS_DENIED is a valid return value but I'd rather have
> some win_skip() or at least a skip() in there. win_skip() will work fine
> right now but as soon as we implement access rights to the registry (and
> multi-user) it should probably be a skip().

That's good feedback, I'll revise the patch.
--Juan