[PATCH] msxml3/tests: Add mxwriter DOMDocument test.

[Jefferson Carpenter]

Let me know what you think about this test.

I could add a double for loop to check over the whole Cartesian product 
of CLSID_MXXMLWriter* and CLSID_DOMDocument* structs.

P.S. We'll see what Marvin has to say about this.  (If Marvin rejects 
it, which he probably will, I'll check support data).