[3/3] user32/tests: Add test for handling WM_PAINT message.

[sir-lexa]

It's my test for situations covered in bugs #2624 and (partly) #3637.
It seems WM_PAINT handling method, used in this test (GetDC-draw-ReleaseDC) is widely used and described in some books (I have one such).
The problem is WM_ERASEBKGND is sent after WM_PAINT thus cleaning custom handler's drawing.
I ran this test on Wine Bot (job 18627) and the results are: on all Windows platforms succeeds, on Wine fails. So I marked it todo.
For other details visit bugs pages - there was a discussion earlier.
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