Today, user32:msg window update region tests may fail if there are
visible topmost windows in the current desktop, which conceals test
windows and thus affects update regions.

Fix this by introducing a temporary desktop to isolate fragile tests in.

Take a precaution to check for unexpected USER environment.  user32
tests are often hard to debug due to its graphical nature, especially in
batch WineTest runs.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=55465
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=55374
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=54202
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=53891