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/builtin.txt b/runtime/doc/builtin.txt
index 7bb48b8..bfb63d0 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -645,6 +645,9 @@
 				none	drop a list of files in a window
 test_gui_mouse_event({button}, {row}, {col}, {repeated}, {mods})
 				none	add a mouse event to the input buffer
+test_gui_tabline_event({tabnr}) Bool	add a tabline event to the input buffer
+test_gui_tabmenu_event({tabnr}, {event})
+				none	add a tabmenu event to the input buffer
 test_ignore_error({expr})	none	ignore a specific error
 test_null_blob()		Blob	null value for testing
 test_null_channel()		Channel	null value for testing