tests: Trigger the widget "update content" code path.

tests/test-naughty-legacy.lua

@@ -1095,6 +1095,11 @@ table.insert(steps, function()
     assert(not n._private.widget_template_failed)
     assert(not n.box)
 
+    -- Check adding messages later
+    n.title   = "foo"
+    n.message = "bar"
+    n.icon    = big_icon
+
     n:destroy()
     handler_called = false
 
@@ -1111,6 +1116,12 @@ table.insert(steps, function()
     assert(had_error)
     assert(not n.box)
 
+    -- Check changing existing content.
+    n.title   = "bar"
+    n.message = "foo"
+    n.icon    = big_icon
+    n.image   = small_icon
+
     handler_called = false
     had_error      = false