ddraw/tests: Skip the primary palette tests if the 640x480x8 mode is not supported.

[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=4809

Your paranoid android.


=== w8 (32 bit ddraw2) ===
ddraw2.c:3231: Test failed: SetDisplayMode failed, hr 0x80004001.