vbscript/tests: Use proper helpers for iface calls

[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=20486

Your paranoid android.


=== WNT4WSSP6 (32 bit createobj) ===
createobj.c:747: Test failed: CoCreateInstance failed: 80040154
createobj: unhandled exception c0000005 at 00402A21