mshtml/tests: Allow ParseUrl to clear buffer on Win10

[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://testbot.winehq.org/JobDetails.pl?Key=23818

Your paranoid android.


=== w1064 (32 bit protocol) ===
protocol.c:723: Test failed: buf changed

=== w1064 (64 bit protocol) ===
protocol.c:723: Test failed: buf changed