msxml3/tests: Constify some character strings

[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=3855

Your paranoid android.


=== wxppro (32 bit domdoc) ===
domdoc.c:9860: Test failed: got 00244F94