To test that GetThemePartSize() should always report theme part size in system DPI if the theme
handle is open without a valid window handle. Previous tests succeeded because the current DPI
happens to be 96.

Tested manually on Windows 10 for 96, 120, 144, 168, 192 and 216 DPI.
Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>