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/src/evalfunc.c b/src/evalfunc.c
index 169f972..627ebb0 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -2330,6 +2330,10 @@
 			ret_void,	    f_test_gui_drop_files},
     {"test_gui_mouse_event",	5, 5, 0,    arg5_number,
 			ret_void,	    f_test_gui_mouse_event},
+    {"test_gui_tabline_event",	1, 1, 0,    arg1_number,
+			ret_bool,	    f_test_gui_tabline_event},
+    {"test_gui_tabmenu_event",	2, 2, 0,    arg2_number,
+			ret_void,	    f_test_gui_tabmenu_event},
     {"test_ignore_error", 1, 1, FEARG_1,    arg1_string,
 			ret_void,	    f_test_ignore_error},
     {"test_null_blob",	0, 0, 0,	    NULL,