Add: WindowDesc unit test to validate ini-key value.

ini-key must be present if WWT_DEFSIZEBOX or WWT_STICKYBOX is present.
This was previously enforced by a workflow, however that parsed the source
code with regex which turned out to be error-prone.

src/window.cpp

@@ -96,7 +96,7 @@ SpecialMouseMode _special_mouse_mode; ///< Mode of the mouse.
  * List of all WindowDescs.
  * This is a pointer to ensure initialisation order with the various static WindowDesc instances.
  */
-static std::vector<WindowDesc*> *_window_descs = nullptr;
+std::vector<WindowDesc*> *_window_descs = nullptr;
 
 /** Config file to store WindowDesc */
 std::string _windows_file;