patch 8.2.4228: no tests for clicking in the GUI tabline

Problem:    No tests for clicking in the GUI tabline.
Solution:   Add test functions to generate the events.  Add tests using the
            functions. (Yegappan Lakshmanan, closes #9638)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 71a4850..66b267f 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1118,6 +1118,8 @@
 	test_getvalue()		get value of an internal variable
 	test_gui_drop_files()	drop file(s) in a window
 	test_gui_mouse_event()	add a GUI mouse event to the input buffer
+	test_gui_tabline_event() add a GUI tabline event to the input buffer
+	test_gui_tabmenu_event() add a GUI tabmenu event to the input buffer
 	test_ignore_error()	ignore a specific error message
 	test_null_blob()	return a null Blob
 	test_null_channel()	return a null Channel