tests: add placeholder for tests on function gui_line_is_action

tests/unit/gui/test-gui-line.cpp

@@ -552,6 +552,16 @@ TEST(GuiLine, HasOfflineNick)
     /* TODO: write tests */
 }
 
+/*
+ * Tests functions:
+ *   gui_line_is_action
+ */
+
+TEST(GuiLine, IsAction)
+{
+    /* TODO: write tests */
+}
+
 /*
  * Tests functions:
  *   gui_line_compute_buffer_max_length