user32: fix SetWindowPos to erase parent when child is hidden

[Clinton Stimpson]

When hiding windows, WM_ERASEBKGND is supposed to be sent to the parent.
The message will now be sent with this patch.  An existing test was 
modified to also check for all parent messages.
Partial fix for bug #6006

Thanks,
Clinton Stimpson

ChangeLog:
  user32: send erase to parent when hiding children


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