Add test for DEBUG_MACRO;...

..."wesnoth --test --debug" should produce a dialog box at start
noting that "Debug mode is on".  Presently this doesn't work (bug
#15204), though the message is emitted just fine if the #ifdef guard
is not there.

data/scenario-test.cfg

@@ -155,6 +155,17 @@ Xu          , Xu          , Qxu         , Qxu         , Ql          , Ql
         [/unit]
     [/side]
 
+#ifdef DEBUG_MODE
+    [event]
+        name=start
+        [message]
+            speaker=narrator
+            icon=wesnoth-icon.png
+            message="Debug mode is on"
+        [/message]
+    [/event]
+#endif
+
 #ifdef EXPERIMENTAL
     [event]
         name=prestart