xmllite/tests: Use an iface instead of a vtbl pointer in testinput.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=7894

Your paranoid android.


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=== W7PRO (32 bit reader) ===
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