mshtml/tests: Fix compilation on systems that don't support nameless unions.

[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
https://newtestbot.winehq.org/JobDetails.pl?Key=2637

Your paranoid android.


=== wxppro (32 bit htmldoc) ===
htmldoc.c:2898: Test failed: mon(0026BF78) != exmon(00269670)
htmldoc.c:7601: Test failed: mon(0026BF78) != exmon(00269670)
htmldoc.c:5978: Test failed: mon(0026BF78) != exmon(00269670)
htmldoc.c:2898: Test failed: mon(002ADFE8) != exmon(00265FE8)
htmldoc.c:7601: Test failed: mon(002ADFE8) != exmon(00265FE8)
htmldoc.c:5978: Test failed: mon(002ADFE8) != exmon(00265FE8)

=== wvista (32 bit htmldoc) ===
No test summary line found

=== w7pro64 (32 bit htmldoc) ===
No test summary line found