kernel32: add more tests for FormatMessage{A,W}

[Louis Lenders]

>FormatMessage uses the same backend to find the requested resource as
>FindResource does, so I'd guess that they both fail in the same way
>when a resource id or a language does not exist in the module.

I still fail to see how I can call FindResource for a missing message-id. In the end they both use LdrFindResource_U but I guess that's not what you meant to test. I still have tthe problem to first load the messagetable with FindResource, and I cannot use a call to FindResource to ask for a present or missing message-id. 

I'll resend the patch with your other two suggenstions, if the patch is not accepted i'll leave it to someone else (more experienced)  to take this up