tests: add testcase for another case of #239

The compositor crashes when a window does:
has shadow -> no shadow -> has shadow

This is because when the shadow is turned off, the shadow image is not
freed, and this is catched by a later assertion.

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

tests/configs/issue239_3.conf

@@ -0,0 +1,4 @@
+shadow = true;
+shadow-exclude = [
+"name = 'NoShadow'"
+]