mlang/tests: Use BSTR strings instead of plain WCHARs when API tells so

[Nikolay Sivov]

On 03/20/2015 09:50 AM, Dmitry Timoshkov wrote:
> The tests were written this way to ensure that mlang methods do not depend
> on BSTR layout (and yes, at least IE6 depends on this).
>

If IE6 abuses it in this way it should have been a separate test with 
explicit comment.