Add automated regression test for CORE-19669

Hey there!

Joachim Henze requested an automated regression test for CORE-19669 in https://github.com/reactos/reactos/pull/7245, so I've spent a bit of time and put something together: scrollbar_redraw.c

It works by creating a window, changing its color any time the WM_SIZE handler detects a change in the window's client area size (it switches through the standard 16 VGA colors available in 16-color mode).

The window also has a timer which triggers a state transition in a state machine every 500ms, and this state machine examines what happens when it makes the scroll bars disappear and re-appear, and it also tries shrinking and growing the window.

The test creates 4 windows, for each possible combination of CS_VREDRAW and CS_HREDRAW.

Could you help me get this added to the test suite?

Thanks,
Marek