tests: add testcase for #239, bug number 2

If a window is unmapped during the draw_callback re-run when the screen
is just redirected, that window won't get a chance to acquire a pixmap.
If fading is enabled, the compositor will try to render that window and
crash.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

tests/configs/issue239_2.conf

@@ -0,0 +1,8 @@
+fading = true;
+fade-in-step = 1;
+fade-out-step = 0.01;
+shadow = true;
+shadow-exclude = [
+"name = 'NoShadow'"
+]
+unredir-if-possible = true;