mshtml/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=20437

Your paranoid android.


=== WVISTAADM (32 bit htmldoc) ===
htmldoc.c:2548: Test failed: unexpected call GetExternal
htmldoc.c:475: Test failed: unexpected riid: {719C3050-F9D3-11CF-A493-00400523A8A0}
htmldoc.c:475: Test failed: unexpected riid: {A0AAC450-A77B-11CF-91D0-00AA00C14A7C}
htmldoc.c:505: Test failed: unexpected call
htmldoc.c:2548: Test failed: unexpected call GetExternal
htmldoc.c:475: Test failed: unexpected riid: {719C3050-F9D3-11CF-A493-00400523A8A0}
htmldoc.c:475: Test failed: unexpected riid: {A0AAC450-A77B-11CF-91D0-00AA00C14A7C}
htmldoc.c:505: Test failed: unexpected call